BENDIX TU-FLO 750 COMPRESSOR User manual

1
®
SD-01-344
DESCRIPTION
The function of the air compressor is to provide and main-
tain air under pressure to operate devices in the air brake
and/or auxiliary air systems. The Tu-Flo®750 compressor
is a two cylinder single stage, reciprocating compressor
with a rated displacement of 16.5 cubic feet per minute at
1,250RPM.
The compressor assembly consists of two major
subassemblies, the cylinder head and the crankcase. The
cylinder head is an iron casting which houses the inlet,
discharge,andunloadervalving.(SeeFigure1.) Thecylinder
head contains the air inlet port and is designed with both
topandside air discharge ports. Three watercoolantports
provide a choice of coolant line connections. Governor
mounting surfaces are provided at both the front and the
rear of the cylinder head. The head is mounted on the
crankcase and is secured by six cap screws. The Tu-Flo®
750 compressor is designed such that the cylinder head
canbeinstalledinoneoftwopositionswhichare180degrees
apart. The crankcase houses the cylinder bores, pistons,
crankshaft and main bearings, and provides the flange or
basemountingsurface.
Bendix®Tu-Flo®750 Air Compressor
BENDIX®TU-FLO®750AIRCOMPRESSOR
(CROSS SECTION)
BENDIX®TU-FLO®750AIRCOMPRESSOR
(EXTERIOR)
AIR DISCHARGE WATER OUTLET
PIECE NO.
TAG
GOVERNOR
MOUNTING
PAD
CRANKCASE
PISTON RINGS
PISTON
CRANKSHAFT
CONNECTING
ROD
INLET VALVE
SEAT
UNLOADER
INLET VALVE
CYLINDER
HEAD
END VIEW OF CYLINDER HEAD
DISCHARGE
VALVE SPRING
DISCHARGE
VALVE
DISCHARGE
VALVE STOP
AIR INLET
CRANKCASE
INLET
INLET VALVE
SPRING
DISCHARGE
VALVE SEAT
UNLOADER
COVER
WATER
INLET
CYLINDER
HEAD

2
OPERATION
Thecompressorisdriven by the vehicleengineandisoper-
ating continuously while the engine is running. Actual
compressionofairiscontrolled by the compressor unload-
ing mechanism and the governor. The governor, which is
generallymountedonthecompressor,maintainsthe brake
system air pressure to a preset maximum and minimum
pressurelevel.
INTAKE AND COMPRESSION OF AIR (LOADED)
During the down stroke of the piston, a slight vacuum is
createdbetweenthetop ofthepistonandthe cylinder head,
causingtheinletvalvetomoveoffitsseatandopen.(Note:
Thedischargevalveremainson itsseat.)Atmospheric airis
drawn through the air strainer and the open inlet valve into
thecylinder(see Figure 4).Asthepiston begins itsupward
stroke,theairthat was drawn into the cylinder on thedown
strokeisbeing compressed.Air pressure on the inlet valve
plustheforceoftheinletspring,returnstheinletvalvetoits
seat and closes. The piston continues the upward stroke
andcompressedairpushesthedischargevalveoffits seat
andairflows bytheopendischarge valve,intothedischarge
lineandtothereservoirs(seeFigure5).Asthepistonreaches
the top of its stroke and starts down, the discharge valve
spring and air pressure in the discharge line returns the
discharge valve to its seat. This prevents the compressed
AIR INLET
WATER
UNLOADER COVER
PLATE
AIR DISCHARGE
WATER
AIR
DISCHARGE
GOVERNOR
MOUNTING
PAD
WATER
MANUFACTURED BY BENDIX
TU-FLO 750 COMPRESSOR
BENDIX NO.
SERIAL NO.
FIGURE 2 - MOUNTING CONFIGURATIONS
Various mounting and drive configurations, as shown in
Figure 2, are supplied as required by the vehicle engine
designs.Anameplateidentifyingthe compressorpiecenum-
ber and serial number is attached to the side of the
crankcase.(ReferenceFigure3.)
FIGURE 3 - NAMEPLATE
FIGURE 1 - CYLINDER HEAD
CAT MACK
(MACK STYLE)
DETROIT
DIESEL
MACK
"FOXHEAD" CUMMINS
MACK
EXTENDED

3
ernorclosesand exhausts the air from abovetheunloader
pistons.Theunloaderspringsforce the pistons upward and
the inlet valves return to their seats. Compression is then
resumed.
COMPRESSOR & THE AIR BRAKE SYSTEM
GENERAL
The compressor is part of the total air brake system, more
specifically, the charging portion of the air brakesystem.As
acomponentintheoverallsystemitscondition, duty cycle,
proper installation and operation will directly affect other
components in the system.
Powered by the vehicle engine, the air compressor builds
the air pressure for the air brake system. The air compres-
sor is typically cooled by the engine coolant system,
lubricated by the engine oil supply and has its inlet con-
nected to the engine induction system.
As the atmospheric air is compressed, all the water vapor
originally in the air is carried along into the air system, as
well as a small amount of the lubricating oil as vapor. If an
air dryer is not used to remove these contaminants prior to
entering the air system, the majority, but not all, will con-
dense in the reservoirs. The quantity of contaminants that
reach the air system depends on several factors including
installation,maintenanceand contaminanthandlingdevices
in the system. These contaminants must either be elimi-
nated prior to entering the air system or after they enter.
GOVERNOR
PORT
AIR
INLET
PORT
INLET
VALVE
OPEN
PISTON
MOVING
DOWN
DISCHARGE
VALVE
CLOSED
GOVERNOR
PORT
INLET
VALVE
CLOSED
PISTON
MOVING
UP
FIGURE 4 - OPERATIONAL-LOADED (INTAKE)
FIGURE 5 - OPERATIONAL-LOADED (COMPRESSION)
GOVERNOR
PORT
DISCHARGE
PORT
AIR
INLET
PORT
FIGURE 6 - OPERATIONAL-UNLOADED
DISCHARGE
VALVE
CLOSED
UNLOADER
PISTON
DISCHARGE
VALVE
OPEN
AIR
INLET
PORT
air in the discharge line from returning to the cylinder bore
as the intake and compression cycle is reseated.
NON-COMPRESSION OF AIR (UNLOADED)
Whenairpressure in the reservoir reaches thecut-out set-
tingofthegovernor, thegovernorallowsairtopassfromthe
reservoir,throughthe governorandintothe cavityabovethe
unloaderpistons.Theunloaderpistonsmove down holding
the inlet valves off their seats (see Figure 6.) With the inlet
valves held off their seats by the unloader pistons, air is
pumpedbackand forth between the two cylinders, and the
dischargevalvesremain closed.Whenairpressurefromthe
reservoirdropsto thecut-insettingofthegovernor,thegov-
INLET VALVE
HELD OPEN
BY UNLOADER
PISTON
AIR
DISCHARGE
PORT
AIR
DISCHARGE
PORT

4
DUTYCYCLE
The duty cycle is the ratio of time the compressor spends
building air to the total engine running time. Air compres-
sorsaredesignedto buildair(run"loaded")up to25%ofthe
time. Higher duty cycles cause conditions that affect air
brakechargingsystemperformancewhichmayrequire ad-
ditional maintenance. Factors that add to the duty cycle
are: air suspension, additional air accessories, use of an
undersizedcompressor,frequentstops,excessiveleakage
from fittings, connections, lines, chambers or valves, etc.
RefertoTableAintheTroubleshootingsectionforaguide to
variousdutycyclesandtheconsiderationthatmustbegiven
tomaintenanceofothercomponents.
COMPRESSORINSTALLATION
While the original compressor installation is usually com-
pletedbythevehiclemanufacturer, conditions of operation
andmaintenancemayrequire additionalconsideration.The
followingpresentsbaseguidelines.
DISCHARGELINE
Thedischargelineallowstheair, water-vapor and oil-vapor
mixture to cool between the compressor and air dryer or
reservoir.Thetypicalsize of a vehicle's discharge line,(see
column2ofTableAintheTroubleshootingsection)assumes
a compressor with a normal (less than 25%) duty cycle,
operating in a temperate climate. See Bendix and/or other
airdryermanufacturerguidelinesasneeded.
FIGURE 6A - SYSTEM DRAWING
AirDryer
Reservoir Drain
Service Reservoir
(Supply Reservoir)
Compressor
Governor
(Governor plus Synchro valve
for the Bendix®DuraFlo™596
Compressor)
Discharge
Line Optional“Ping”Tank
OptionalBendix®PuraGuard®QC™
OilCoalescingFilter
The Air Brake Charging System supplies the
compressedairforthebrakingsystemaswellasotherair
accessories for the vehicle. The system usually consists
of an air compressor, governor, discharge line, air dryer,
andservicereservoir.
The discharge line must maintain a constant slope down
fromthecompressorto the air dryer inlet fitting or reservoir
toavoidlow pointswhereicemay formandblocktheflow. If,
instead, ice blockages occur at the air dryer or reservoir
inlet,insulation may be addedhere, or if the inlet fitting is a
typical 90 degree fitting, it may be changed to a straight or
45 degree fitting. Shorter discharge line lengths or insula-
tion may be required in cold climates.
While not all compressors and charging systems are
equippedwithadischargelinesafetyvalvethiscomponent
isrecommended.The dischargelinesafetyvalve isinstalled
inthe cylinder head (Tu-Flo®550/750)or close to the com-
pressordischargeportandprotectsagainstoverpressurizing
thecompressorinthe event of a discharge line freezeup.
FIGURE 6B - DISCHARGE LINE SAFETY VALVE
HOLE
THREAD

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COOLING
Airflowingthroughtheengine compartment from the action
oftheengine’sfanandthemovementofthevehicleassists
incoolingthecompressor.Coolantflowingfromtheengine’s
cooling system through connecting lines enters the head
andpassesthroughinternal passages in the cylinder head
andisreturnedtotheengine.Propercoolingisimportantin
maintainingdischargeair temperaturesbelowthemaximum
recommended400degreesFahrenheit.
Figure 8 illustrates the various approved coolant flow con-
nections. See the tabulated technical data in the back of
thismanualforspecificrequirements.
AIR INDUCTION
Therearethreemethodsofproviding cleanairtotheTu-Flo®
750compressor:
1. Naturally aspirated Local Air Strainer - Compressor
utilizesitsownattachedairstrainer(polyurethanesponge
orpleatedpaperdryelement).
2. Naturally aspirated Engine Air Cleaner - Compressor
inletisconnectedtotheengineaircleanerorthevacuum
side (engine air cleaner) of the supercharger or
turbocharger.
3. Pressurized induction - Compressor inlet is connected
tothepressuresideofthe superchargerorturbocharger.
Seethe tabulated technical data in the backof this manual
forspecificrequirementsfornumbers2and3above.
DISCHARGELINE TEMPERATURE
When the temperature of the compressed air that enters
the air dryer is within the normal range, the air dryer can
removemostof the charging system oil. Ifthe temperature
ofthe compressed air is abovethe normal range, oil as oil-
vapor is able to pass through the air dryer and into the air
system.Largerdiameterdischarge lines and/or longer dis-
chargelinelengthscanhelp reduce the temperature.
Theairdryercontainsafilterthatcollects oil droplets, and a
desiccantbedthatremovesalmostalloftheremainingwa-
tervapor.Thecompressedairisthenpassedtotheairbrake
service (supply) reservoir. The oil droplets and the water
collected are automatically purged when the governor
reaches its "cut-out" setting.
For vehicles with accessories that are sensitive to small
amounts of oil, we recommend installation of a Bendix®
PuraGuard®QC™oilcoalescingfilter,designedtominimize
the amount of oil present.
LUBRICATION
The vehicle's engine provides a continuous supply of oil to
the compressor. Oil is routed from the engine to the com-
pressoroilinlet.An oilpassageinthecompressorcrankshaft
allows oil to lubricate the connecting rod crankshaft bear-
ings.Connectingrodwristpinbushingsandcrankshaftball
bearings are spray lubricated.An oil return line connected
fromthecompressordrainoutlettothevehicleenginecrank-
caseallowsforoil return.Onflangemountedmodels,the oil
drains back directly to the engine through the mounting
flange.
FIGURE 7 - LUBRICATION (BASE MOUNT MODEL SHOWN)
OIL
INLET
OIL
OUTLET
WATER
IN
OR
(1 PORT
ONLY)
WATER
OUT
OR
(1 PORT
ONLY)
WATER OUT
WATER
OUT
WATER
IN
WATER
IN
FIGURE 8 - COOLING

6
If a previously unturbocharged compressor is being turbo-
charged, it is recommended that the inlet cavity screen
(238948)beinstalledwithaninletgasket(291909)on both
sides of the screen.
COMPRESSOR TURBOCHARGING
PARAMETERS
Air entering the compressor inlet during the loaded cycle
mustnotexceed 250degreesFahrenheit(121degreesCel-
sius). A metal inlet line is suggested to help meet this
parameter.
Thefollowingcompressorcrankshaftrotative speed and in-
let pressure relationships may not be exceeded.
Crankshaft Maximum CompressorR.P.M.
Inlet Pressure
1900RPM 30.0 psi (207 kPa)
2000RPM 27.5 psi (190 kPa)
2100RPM 24.0 psi (165 kPa)
2200RPM 21.0 psi (145 kPa)
2300RPM 19.0 psi (131 kPa)
2400RPM 16.0 psi (110 kPa)
AIR INDUCTION
One of the single most important aspects of compressor
preventive maintenance is the induction of clean air. The
typeandintervalofmaintenancerequiredwillvarydepending
upon the air induction system used.
Theintervalslisted undertheheadingsbelow pertaintotypi-
calhighwayandstreetoperation.Morefrequentmaintenance
willberequiredforoperationindustyor dirty environments.
0
20
25
15
35
5
30
270024002100180015001200900600
10
CompressorSpeed(RPM)
InletPressure(PSIG)
TurboLimits
FIGURE 9 - TURBO LIMITS CURVE
PREVENTATIVE MAINTENANCE
Regularlyscheduledmaintenanceisthe single most impor-
tant factor in maintaining the air brake charging system.
RefertoTableAin theTroubleshooting sectionforaguideto
various considerations that must be given to the mainte-
nanceofthecompressor andotherrelatedchargingsystem
components.
ImportantNote:Reviewthewarrantypolicybeforeperform-
ing any intrusive maintenance procedures. An extended
warrantymaybevoidedifintrusivemaintenanceisperformed
duringthisperiod.
PAPERAIR STRAINERDRYELEMENT-PLEATED
POLYURETHANE SPONGESTRAINER
FIGURE 10 - STRAINERS
POLYURETHANE SPONGE STRAINER
Everymonth,150 operatinghours or5,000miles, which-
ever occurs first, remove and wash all of the parts. The
strainer element should be cleaned or replaced. If the ele-
mentis cleaned,itshouldbewashedinacommercialsolvent
or a detergent and water solution. The element should be
saturatedinclean engine oil, then squeezed drybeforere-
placing it in the strainer. Be sure to replace the air strainer
gasketiftheentire strainer is removedfromthecompressor
intake.

7
DRY ELEMENT - PLEATED PAPER STRAINER
Every two months, 800 operating hours or 20,000 miles
whichever occurs first, loosen the spring clip from the
unhinged side of the mounting baffle and open the cover.
Replacethepleatedpaperfilterandsecurethecleanedcover,
making sure the filter is in position. Be sure to replace the
air strainer gasket if the entire air strainer is removed from
thecompressor intake.
INTAKEADAPTER
When the engine air cleaner is replaced: Some com-
pressorsarefittedwith compressor intake adapters, which
allowthecompressor intake to be connected totheengine
airinductionsystem.Inthis case, the compressor receives
a supply of clean air from the engine air cleaner. When the
engineair filter is changed, the compressor intake adapter
shouldbechecked.Ifitisloose,removetheintakeadapter,
cleanthestrainerplate,if applicable, and replace theintake
adapter gasket, and reinstall the adapter securely. Check
lineconnectionsbothatthe compressor intake adapter and
at the engine. Inspect the connecting line for ruptures and
replace it if necessary.
COMPRESSOR COOLING
Every 6 months, 1,800 operating hours or after each
50,000 miles whichever occurs first, inspect the com-
pressor discharge port, inlet cavity and discharge line for
evidenceofrestrictionsandcarboning.Ifexcessive buildup
isnoted,thoroughlycleanorreplacetheaffectedpartsand
closely inspect the compressor cooling system. Check all
compressor coolant lines for kinks and restrictions to flow.
Minimum coolant line size is 3/8" I.D. Check coolant lines
forinternalclogging from rust scale. If coolant lines appear
suspicious,checkthecoolant flowandcomparetothe tabu-
lated technical data present in the back of this manual.
Carefully inspect the air induction system for restrictions.
LUBRICATION
Everysixmonths, 1,800operatinghours or50,000miles
whichever occurs first, check external oil supply and re-
turn lines, if applicable, for kinks, bends, or restrictions to
flow. Supply lines must be a minimum of 3/16" I.D. and
return lines must be a minimum of 1/2" I.D. Oil return lines
should slope as sharply as possible back to the engine
crankcase and should have as few fittings and bends as
possible. Refer to the tabulated technical data in the back
ofthismanualfor oil pressure minimum values.
Checktheexteriorofthecompressorforthe presence of oil
seepageandrefertothe TROUBLESHOOTING section for
appropriatetestsandcorrectiveaction.
OIL PASSING
All reciprocating compressors currently manufactured will
pass a minimal amount of oil. Air dryers will remove the
majority of oil prior to entrance into the air brake system.
ForparticularlyoilsensitivesystemstheBendix®PuraGuard®
QC™oil coalescing filter can be used in conjunction with a
Bendixairdryer.
If compressor oil passing is suspected, refer to the
TROUBLESHOOTINGsectionandTABLEAforthe symp-
toms and corrective action to be taken. In addition, Bendix
has developed the "Bendix Air System Inspection Cup" or
BASIC test to help substantiate suspected excessive oil
passing. The steps to be followed when using the BASIC
test are presented in APPENDIX A at the end of the
TROUBLESHOOTINGsection.
COMPRESSOR DRIVE
Everysix months,1,800operating hoursor 50,000miles,
whichever occurs first, check for noisy compressor
operation,whichcouldindicateaworndrivegearcoupling,
a loose pulley or excessive internal wear. Adjust and/or
replace as necessary.
If the compressor is belt driven, check for proper belt and
pulley alignment and belt tension. Check all compressor
mountingboltsandretightenevenlyif necessary.Checkfor
leakageandproperunloadermechanism operation. Repair
or replace parts as necessary.
Every 24 months, 7,200 operating hours, or after each
200,000miles,perform athoroughinspection,anddepend-
ing upon the results of this inspection or experience,
disassemble the compressor, clean and inspect all parts
thoroughly, replace all worn or damaged parts using only
genuine Bendix replacements or replace the compressor
withagenuineBendixremanufacturedunit.
GENERAL SERVICE CHECKS
OPERATING TESTS
Vehicles manufactured after the effective date of FMVSS
121,withtheminimumrequiredreservoirvolume,musthave
a compressor capable of raising air system pressure from
85-100 psi in 25 seconds or less. This test is performed
withtheengineoperating at maximum recommended gov-
erned speed. The vehicle manufacturer must certify this
performanceon new vehicles withappropriate allowances
for air systems with greater than the minimum required
reservoirvolume.

8
AIR LEAKAGE TESTS
Compressorleakagetestsneednotbeperformedonaregular
basis. These tests should be performed when; it is sus-
pectedthatdischargevalveleakageissubstantiallyaffecting
compressorbuild-up performance, or when itis suspected
that the compressor is “cycling” between the load and un-
loadedmodesduetounloaderpistonleakage.
Thesetestsmustbe performed withthevehicleparkedon a
level surface, the engine not running, the entire air system
completely drained to 0 P.S.I., and the inlet check valve
detailpartsremoved,ifapplicable.
UNLOADER PISTON LEAKAGE
The unloader pistons can be checked for leakage as fol-
lows:withthecylinder head removed from the compressor
and the inlet flange securely covered, apply 120 psi of air
pressuretothegovernor port. Listen for an escape of airat
the inlet valve area.An audible escape of air should not be
detected.
DISCHARGE VALVE LEAKAGE
Unloaderpistonleakagemust be repaired beforethistestis
performed. Leakage past the discharge valves can be de-
tectedasfollows:Removethedischargelineandapplyshop
airbackthroughthedischargeport.Listenforanescapeof
airatthecompressor inlet cavity.Abarely audible escapeof
airis generally acceptable.
Ifthecompressordoes not functionasdescribedaboveorif
the leakage is excessive, it is recommended that it be re-
turned to the nearest authorized Bendix distributor for a
factoryremanufacturedcompressor.Ifitisnotpossible,the
compressor can be repaired using a genuine Bendix cylin-
der head maintenance kit. Retest the cylinder head after
installation of the kit.
REMOVAL AND DISASSEMBLY
GENERAL
Thefollowingdisassemblyandassemblyprocedureispre-
sentedforreferencepurposesandpresupposesthatamajor
rebuildofthecompressorisbeingundertaken.Severalmain-
tenance kits are available which do not require full
disassembly.Theinstructions providedwiththesepartsand
kits should be followed in lieu of the instructions presented
here.
REMOVAL
These instructions are general and are intended to be a
guide, in some cases additional preparations and precau-
tions are necessary.
1. Block the wheels of the vehicle and drain the air pres-
sure from all the reservoirs in the system.
2. Drain the engine cooling system and the cylinder head
ofthecompressor.Identifyand disconnect all air, water
and oil lines leading to the compressor.
3. Removethegovernorandany supporting bracketry at-
tached to the compressor and note their positions on
the compressor to aid in reassembly.
4. Remove the discharge and inlet fittings, if applicable,
and note their position on the compressor to aid in
reassembly.
5. Removetheflangeorbasemountingboltsand remove
thecompressorfromthevehicle.
6. Removethedrivegear(s)or pulleyfromthecompressor
crankshaftusing a gearpuller.Inspectthepulleyorgear
andassociatedpartsfor visible wear or damage. Since
thesepartsareprecisionfitted,theymustbereplacedif
theyarewornordamaged.
PREPARATION FOR DISASSEMBLY
Remove road dirt and grease from the exterior of the com-
pressor with a cleaning solvent. Before the compressor is
disassembled,thefollowingitemsshould bemarkedtoshow
theirrelationshipwhenthecompressorisassembled.Mark
the rear end cover in relation to the crankcase. Mark the
base plate or base adapter in relation to the crankcase.
Aconvenientmethod to indicate the above relationshipsis
to use a metal scribe to mark the parts with numbers or
lines. Do not use marking methods such as chalk that can
bewipedofforobliteratedduring rebuilding.
CYLINDER HEAD
Remove the six cylinder head cap screws (1) and tap the
head with a soft mallet to break the gasket seal. Remove
the unloader cover plate cap screws (2), lockwashers (3)
and the unloader cover plate (4). Scrape off any gasket
material (5) from the cover plate, cylinder head and crank-
case.
1. Removetheunloaderpistons(7),o-rings(6) andsprings
(8).
2. Inspecttheunloaderpiston bushings (9)fornicks,wear,
corrosionandscoring.Itisrecommendedthatthecom-
pressorbereplacedifitisdeterminedthattheunloader
bushingisdamagedorwornexcessively.
Beforedisassemblingthedischargevalvemechanism,mea-
sure and record the discharge valve travel (from closed to
completelyopen).
3. If the measured discharge valve travel exceeds .046
inches, the compressor should be replaced. If the
discharge valve travel does not exceed .046, using a
9/16" Allen wrench, remove the discharge valve seats
(18),valves(17) andvalvesprings(16).

9
4. Removetheinletvalvestops(14),valves(17),valveseats
(11), valve springs (12) and gaskets (10). It is recom-
mended that a tool such as a J-25447-B, produced by
Kent Moore Tool Division Roseville, Michigan phone
1-800-328-6657,beusedto removetheinletvalvestop.
CRANKCASE BOTTOM COVER OR ADAPTER
DISASSEMBLY
1. Removethecapscrews(22)securingthebottomcover
or adapter (21). Tap with a soft mallet to break the gas-
ket seal. Scrape off any gasket material (20) from the
crankcaseandbottomcoveroradapter.
CONNECTING ROD DISASSEMBLY
Before removing the connecting rod, mark the connecting
rods(37)andtheircaps(39)toensurecorrectreassembly.
Theconnectingrod andcapareamatched setthereforethe
caps must not be switched or rotated end for end.
1. Removetheconnectingrodbolts (40) and bearingcaps
(39).
2. Push the pistons (26) with the connecting rods (37) at-
tachedoutthetopofthecylinderboreof the crankcase.
Replacethe bearing caps on theconnecting rods.
3. Remove the piston rings (23-25) from the piston. If the
pistonisto beremovedfromthe connectingrod,remove
the wrist pin teflon plugs (28) and press the wrist pin
(27)fromthepiston and connecting rod.
4. If the piston is removed from the rod, inspect the wrist
pinborein the piston and bronze wristpinbushing (36)
intheconnectingrod.Ifexcessivewearisnotedorsus-
pected,replace the connecting rod andpiston.
COMPRESSOR CRANKCASE DISASSEMBLY
1. Remove the key or keys (30) from the crankshaft (29)
andanyburrsfrom thecrankshaftwherethekey orkeys
wereremoved.(Note:Throughdrivecompressorsmay
have a crankshaft key at both ends.)
2. Remove the four cap screws (35) and lockwashers or
nuts and lockwashers that secure the rear end cover
(34) to the crankcase.
3. Removetherearendcover (34), thrust washer(31)and
end cover oil seal ring (33), taking care not to damage
thebearingifpresentin the end cover.
4. If the compressor has ball type main bearings, press
the crankshaft (29) and ball bearings from the crank-
case, then press the ball bearings from the crankshaft.
5. Press the oil seal out of the compressor crankcase, if
soequipped.
CLEANING OF PARTS GENERAL
All parts should be cleaned in a good commercial grade of
solventanddriedpriortoinspection.
CYLINDER HEAD
Removecarbondepositsfrom the dischargecavityandrust
andscalefromthecoolingcavitiesofthecylinderheadbody.
Scrape all foreign matter from the body surfaces and use
shopairpressuretoblowthedirt particles from the cavities.
Cleancarbonand dirtfromtheinletand unloaderpassages.
Use shop air to blow the carbon and dirt deposits from the
unloaderpassages.
OIL PASSAGES
Thoroughly clean all oil passages through the crankshaft,
crankcase,endcovers,baseplateorbase adapter. Inspect
the passages with a wire to be sure. Blow the loosened
foreignmatteroutwithair pressure.
INSPECTION OF PARTS
CYLINDER HEAD BODY
Inspect the cylinder head for cracks or damage. With the
cylinder head and head gasket secured to a flat surface or
crankcase, apply shop air pressure to one of the coolant
ports with all others plugged, and check for leakage by ap-
plyingasoap solution to the exterior of the body. If leakage
is detected, replace the compressor.
END COVERS
Check for cracks and external damage. If the crankshaft
main bearing (32) is installed in the end cover (34), check
forexcessive wear and flatspotsandreplaceifnecessary.
CRANKCASE
Check all crankcase surfaces for cracks and damage. On
compressorswhereball bearingmainbearingsare usedthe
differencebetweentheO.D. of theouterraceandtheI.D. of
the crankcase hole should be .0003 in. tight to .0023 in.
loose. This is to maintain the correct fit. The compressor
must be replaced if the fit is too loose.
Oncompressorsfittedwith precision,sleevemainbearings,
the difference between the O.D. of the crankshaft journal
and the main bearing l.D. must not exceed .005 in. If the
clearance is greater than .005 in. the bearing must be re-
placed.
Thecylinderboresshouldbe checked with insidemicrome-
ters or calipers. Cylinder bores which are scored or out of
roundbymorethan.0005 in. ortaperedmorethan.0005in.

10
ITEM QTY DESCRIPTION ITEM QTY DESCRIPTION ITEM QTY DESCRIPTION
1 6 Cylinder Head Cap Screws 15 2 DischargeValveStop 29 1 Crankshaft
2 4 UnloaderPlateCapScrews 16 2 DischargeValveSpring 30 1 CrankshaftKey
3 4 UnloaderPlateLockWashers 17 2 DischargeValve 31 2 ThrustWasher
4 1 UnloaderPlate 18 2 DischargeValveStop 32 2 Sleeve(orBall)Bearing
5 1 UnloaderPlateGasket 19 1 CylinderHeadGasket 33 1 End Cover Seal
6 2 O-ring 20 1 BaseGasket 34 1 End Cover
7 2 Unloader 21 1 BasePlate 35 4 End Cover Cap Screws
8 2 Spring 22 6 Base Plate Cap Screws 36 2 WristPinBushing
9 2 UnloaderBushing 23 6 StandardPistonRings 37 2 ConnectingRod
10 2 Gasket 24 8 OilRing 38 2 Conn.RodInserts(Sets)
11 2 InletValveSeat 25 4 ExpanderRing 39 2 ConnectingRodCaps
12 2 InletValve 26 2 Piston 40 4 ConnectingRodBolts
13 2 InletValveSpring 27 2 WristPin
14 2 InletValveStop 28 4 WristPinButton
31
1
2
3
4
5
6
7
8
9
23
23
24
25
24
28
27
23
35
25
24
10
11
12
13
24
37
14
36
33 32
34
29
30
38
39
40
NAMEPLATE
20
21
22
15
16
17
18
19
CRANKCASE
32
26
31
CYLINDER
HEAD
32
BALL BEARING
(MACK EXTENDED
FLANGE)
FIGURE 11 - EXPLODED VIEW

11
shouldbereboredorhonedoversize.Oversizedpistonsand
piston rings are available in .010 in., .020 in. and .030 in.
oversizes. Cylinder bores must be smooth, straight and
round. Clearance between the cast iron pistons and cylin-
derboresshouldbebetween.002in. minimum and .004 in.
maximum.
PISTON RINGS
Checkthepistonsforscores,cracksorenlargedringgrooves;
replace the pistons if any of these conditions are found.
Measure each piston with a micrometer in relation to the
cylinderborediametertobesurethediametricalclearance
is between .002 in. minimum and .004 in. maximum.
Checkthefitof the wrist pins to the pistons andconnecting
rodbushings. The wrist pinshould be a lightpress fit in the
piston. If the wrist pin is a loose fit, the piston and pin as-
sembly should be replaced. Check the fit of the wrist pin in
theconnectingrod bushingbyrockingthe piston.Thisclear-
ance should not exceed .0007 in. Replace the connecting
rod and cap assembly which includes the wrist pin bush-
ings if excessive clearance is found. Check the fit of the
ringsinthepistonringgrooves.Checktheringgapwiththe
rings installed in the cylinder bores. Refer to Figure 12 for
correctgapandgrooveclearances.
CRANKSHAFT
Checkthecrankshaft threads, keyways, tapered ends and
allmachinedandgroundsurfaces for wear,scores,ordam-
age.Standardcrankshaftjournalsare1.1242 in.-1.1250in.
indiameter. Ifthecrankshaftjournalsareexcessivelyscored
or worn or out of round and cannot be reground, the com-
pressor must be replaced. Connecting rod bearing inserts
areavailablein.010in.,.020in.and.030in.undersizes for
compressorswithregroundcrankshafts.Mainbearing jour-
nals must be maintained so the ball bearings are a snug fit
or so that no more than .005 in. clearance exists between
theprecisionsleevemainbearingandthemainbearingjour-
nals on the crankshaft. Check to be sure the oil passages
areopenthroughthecrankshaft.
CONNECTING ROD BEARINGS
Usedbearinginsertsmustbe replaced. The connectingrod
andcapareamatchedsetandthereforethecapsmustnot
be switched or rotated end for end. Make sure the locating
tangsontheinsertsengagewiththelocating notches in the
rodandcap.Clearancebetween the connecting rodjournal
andtheconnectingrodbearingmust not be lessthan.0003
in.ormore than .0021 in. after rebuilding.
REPAIRS
UNLOADER
Anewcylinderheadmaintenancekitshouldbeusedwhen
rebuilding. Note: The entire contents of this kit must be
used. Failure to do so may result in compressor fail-
ure. Theunloaderpistonsinthekitareprelubricatedwitha
special lubricant piece number 239379 and need no addi-
tionallubrication.Installthespringsandunloaderpistons in
their bores being careful not to cut the o-rings. Install the
unloader cover gasket and unloader cover and secure the
cover cap screws. Tighten the cap screws to 175-225 in.
Ibs. in a crossing pattern after first snugging all screws.
DISCHARGE VALVES, VALVE STOPSAND
SEATS
Ifthedischargevalveseatsmerely showsignsof slightwear,
they can be dressed by using a lapping stone, grinding
compoundandgrindingtoolhoweveritisrecommended that
a cylinder head maintenance kit be used. Install new
dischargevalvespringsandvalves.Screwin the discharge
valveseats,andtightento70-90ft.-lbs.Dischargevalvetravel
shouldbebetween.030in.to.046in.Totestfor leakage by
the discharge valves, apply 100 psi to the cylinder head
discharge port and apply a soap solution to the discharge
valve and seats. Leakage in the form of soap bubbles is
permissible. If excessive leakage is found, leave the air
pressure applied and with the use of a fiber or hardwood
dowelandahammer, tapthedischargevalvesofftheirseats
several times. This will help the valves to seat and should
reducetheleakage.Withtheairpressurestillapplied at the
dischargeportofthecylinderhead,checkforleakagearound
the discharge valve stop on the top of the cylinder head
casting. No leakage is permitted.
SIDE CLEARANCE
.000
.006
STANDARD
PISTON
RING
.002
.004 OIL RING
EXPANDER
RING
END GAP
.002
.013
.010
.040
Segment
Ring End
Gap
FIGURE 12 - RING CONFIGURATION
Compression

12
INLET VALVESAND SEATS
Inletvalvesandspringsshould be replaced. However,ifthe
inlet valve seats show signs of slight nicks or scratches,
theycanberedressedwithafinepieceofemery cloth or by
lappingwithalappingstone,grindingcompound and grind-
ing tool. If the seats are damaged to the extent that they
cannot be reclaimed, they must be replaced.
ASSEMBLY
General Note:All torques specified in this manual are as-
sembly torques and typically can be expected to fall off
afterassemblyis accomplished. Do not retorque after ini-
tial assembly torques fall unless instructed otherwise. A
compiledlistingof torque specifications is presented at the
endof this manual.
To convert inch pounds of torque to foot pounds of torque,
divideinchpoundsby12.
inch pounds ÷ 12 = foot pounds
To convert foot pounds of torque to inch pounds of torque,
multiply foot pounds by 12.
foot pounds x 12 = inch pounds
INSTALLING CRANKSHAFT
Pressnewsleevebearings in theendcoverandcrankcase.
Ensure that the slot in the bearings line up with the oil pas-
sages in the end cover or crankcase. If you have a model
with no oil passage present in the crankcase, press the
sleeve bearing into the crankcase with the slot located 90
degreesfromvertical.
Install the front thrust washer with the tang inserted in
the slot toward the flange. Insert the crankshaft and the
rear thrust washer with the tang toward the rear of the
compressor.
Placetheoilsealringonthebossoftherearendcoverand
installtheendcover making sure not to pinch the sealring.
Ensure the tang of the thrust washer is inserted in the slot
oftheend cover.Fastentheendcover tothecrankcasewith
the four cover cap screws. Torque the cap screws to 175-
225 inch pounds in a cross pattern.
PISTONS AND CONNECTING RODS
If the pistons are to be replaced ensure that the correct
pistons are being installed. Note that the pistons for the
Tu-Flo®750compressoraresimilar tothoseofotherBendix
compressor models but may be identified by the piston
diameterandthedistancetothecenterofthe wrist pin from
the top of the piston as shown in Figure 13.
PISTON RINGS
Check each ring end gap in a cylinder bore before installa-
tion.Placethe ring in the top ofthecylinder bore and using
thepiston,pushtheringtothemidpointofthecylinderbore
and check the ring gap. If the end gaps are incorrect either
thewrongrepair size has been purchased or the compres-
sorisworn beyond specification and should bereplaced.
FIGURE 13 - PISTON COMPARISON
2.78
2.17
1.06
1.25
OTHER BENDIX®
TU-FLO®AIR COMPRESSORS TU-FLO®750 AIR COMPRESSOR
PISTON COMPARISON
Installtheringsonthepistonsperthefollowinginstructions
startingat the center of the pistonandmoving outward.
1. Install the spacer and segment rings as follows. Place
thespacerring(25)inthe piston groove, theendsofthe
spacer must butt and not overlap. Install the top seg-
ment (24) by inserting one end above the spacer in the
ringgroove,120degreesfromthespacerendsandwind
thesegmentintoposition.Installthebottomsegmentin
thesamemanner beneath the spacer, making sure the
gap is staggered 120 degrees from both the top ring
segmentandthe spacerendgaps.Beforeusing besure
both painted ends of the spacer are visible and butted.
(RefertoFigure14.)
FIGURE 14 - PISTON & RINGS
COMPRESSION RING (23)
SEGMENT RING (24)
SPACER RING (25)
SEGMENT RING (24)

13
2. Installthecompressionrings(23)inthepropergrooves
withthebevelor“pip”mark(ifany)towardthetopof the
piston.(RefertoFigure14.)
Checktheringsideclearanceofeachringinthepistonring
groove. (Refer to Figure 12.) If the side clearance is too
large, the piston ring groove is worn beyond specifications
and the piston must be replaced.
Rotate the piston rings in their respective groove so that
eachendgapisat least 90degreesfromthepreviousring’s
endgap.
Lubricatethewristpin(22)andwrist pin bushing in the con-
necting rod with engine oil.Assemble the upper portion of
the connecting rods and the pistons with the wrist pins.
Insert the wrist pin buttons (28) in the ends of the wrist pin.
Lubricatethepistonsandringswithengineoil.Usingaring
compression tool return the piston to the cylinder bore.
Turn the crankshaft so that one of its connecting rod jour-
nalsisinthedownward,centerposition.Installthecrankshaft
journal bearing segments (38) on the connecting rod (37)
and connecting rod cap (39). Tighten the connecting rod
bolts (40) evenly and torque to 150 - 170 inch pounds. In-
stalltheotherconnectingrodandpistoninthesamemanner.
It is recommended that new connecting rod cap screws be
used.
Beforereplacingthecylinder headonthecrankcaseensure
the correct pistons have been used by turning the crank-
shaftonecompleterevolutionsuchthateachpistonmoves
to its maximum upward stroke. At the maximum upward
stroke position each piston should move to the top of the
crankcase. If the piston does not approach the top of the
crankcasethepistonisincorrect,andif not replaced, could
resultincompressordamage.
BASE PLATE OR BASE ADAPTER
Position the base plate or base adapter gasket (20) on the
crankcase and install the base plate or base adapter (21)
asmarkedbefore disassembly.Tighten the six cap screws
(22),securingthecastironbaseadapterevenlytoatorque
of175-225inchpounds for baseplateorcoverina crossing
pattern after first snugging all 6 screws.
CYLINDER HEAD
Place the cylinder head gasket (19) and cylinder head on
thecompressorcrankcaseandinstall the six cylinder head
cap screws. Snug the cylinder head cap screws prior to
torquing the cap screws to 300-360 in. Ibs. in a cross pat-
tern.Retorquethe unloadercovercapscrewsto170-225in.
Ibs.
FINAL COMPRESSOR ASSEMBLY
Install all crankshaft keys making certain to support the
crankshaft to avoid bearing damage. Install the crankshaft
nutwhereapplicable.Wheninstallingdrivecouplingsorgears,
do not exceed 120 foot pounds torque onthecrankshaft
nut.
Use covers, plugs, or masking tape to protect all ports if
compressor is not to be installed immediately. Protect the
ends of the crankshaft against damage by wrapping with
masking tape or friction tape.
TESTING REBUILT COMPRESSOR
Inordertoproperlytestacompressorunderoperatingcon-
ditions,atestrackfor correct mounting,cooling,lubricating,
and driving the compressor is necessary. Such tests are
not compulsory if the unit has been carefully rebuilt by an
experienced person. A compressor efficiency or build up
test can be run which is not too difficult. An engine lubri-
cated compressor must be connected to an oil supply line
ofatleast15P.S.I.pressure during thetestandanoilreturn
line must be installed to keep the crankcase drained.
Connecttothecompressordischargeport,areservoirwith
avolumeof1,500cubicinches,includingthevolumeofthe
connecting line. With the compressor operating at 2,100
R.P.M.,the time required to raise the reservoir(s) pressure
from85psito100psishouldnotexceed5seconds.During
thistest,the compressorshouldbecheckedfor gasketleak-
ageandnoisyoperation,aswellasunloaderoperationand
leakage.
If the compressor functions as indicated reinstall on the
vehicle connecting all lines as marked in the disassembly
procedure.
TU-FLO®750 AIR COMPRESSOR
SPECIFICATIONS
Averageweight ............................................................ 53
Numberofcylinders ...................................................... 2
Bore size ............................................................. 2.78 In.
Stroke .................................................................. 1.87 In.
Displacementat1250 RPM.............................. 16.5 CFM
CFMMaximumrecommendedRPM .............. 2,400 RPM
Minimumcoolantflow(watercooled)at
MaximumRPM ................................................. 2.5 GPM
MinimumRPM ..................................................... 5 GPM
Approximatehorsepowerrequiredat
1250RPMat120PSIG (naturally aspirated) .............. 3.2
Turbochargelimits
SeeCompressorTurbocharging Parameters
Maximuminletairtemperature ............................... 250°F
Maximumdischargeairtemperature ...................... 400°F
Minimumpressurerequiredtounload
(naturallyaspirated)............................................ 60 PSIG

14
Minimumoilpressurerequiredat
engineidlingspeed ............................................ 15 PSIG
Minimumoilpressurerequiredat
maximumgovernedengine speed ...................... 15 PSIG
Minimumdischarge-linesize .............................. 1/2" I.D.
Minimumcoolant-linesize.................................. 3/8" I.D.
Minimum oil-supply line size ............................. 3/16" I.D.
Minimumoil-returnline size................................ 1/2" I.D.
Minimum air-inlet line size....................................5/8” I.D.
Minimumunloader-linesize............................... 3/16" I.D.
TORQUE SPECIFICATIONS
Bolt, Nut or Screw ............................ Assembly Torque
(in. Ibs.)
CylinderHead ....................................................300 - 360
UnloaderCoverPlate......................................... 175 - 225
DischargeValveSeat .............. 840 - 1,080 (70-90 ft. Ibs.)
InletValveStop........................ 840 - 1,080 (70-90 ft. Ibs.)
EndCover ......................................................... 175 - 225
ConnectingRod ................................................ 150 - 170
BottomCover .................................................... 175 - 225
AirStrainer ........................................................ 125 - 150
InletFitting ........................................................ 175 - 225
Discharge Fitting ............................................... 175 - 225
Governoror GovernorAdapter ............................ 175 - 225
Pipe Plugs
1/16 ..................................................................... 35 - 50
1/8 ..................................................................... 85 - 105
1/4 .................................................................... 130 - 170
3/8 .................................................................... 160 - 200
1/2 .................................................................... 200 - 270
PipeBushing
1/2 .................................................................... 175 - 225
CrankshaftNut:
MarsdenorCastle............ 1,200 - 1,400 (100-120 ft. lbs.)
P/N298125
(MetricThread)...................2,640-3,048(220-254ft.lbs.)
DIMENSIONAL DATA
Port Sizes
Water inlet ................................................... 1/2 - 14 NPT
Wateroutlet ................................................. 1/2 - 14 NPT
Airdischarge................................................ 1/2 - 14 NPT
Governor ...................................................... 1/8 - 27 NPT
Oilinlet(endcover) ...................................... 1/8 - 27 NPT
Piston
(standard)....................................................... 2.77825 in.
(.010oversize)................................................ 2.78825 in.
(.020oversize)................................................ 2.79825 in.
(.030oversize)................................................ 2.80825 in.
Cylinderbore
(standard)......................................................... 2.7810 in.
(.010oversize).................................................. 2.7910 in.
(.020oversize).................................................. 2.8010 in.
(.030oversize).................................................. 2.8110 in.
MAINTENANCE KITSANDAVAILABLE SERVICE
PARTS
CylinderMaintenanceKit.
PistonRingKit(standard and oversizes.)
PistonandRodKit (standard and oversizes.)
CrankshaftBearingKit.
Gasket & Seal Kit.
COMPRESSOR TROUBLESHOOTING
IMPORTANT: Thetroubleshootingcontainedinthissection
considers the compressor as an integrated component of
theoverallair brake charging system and assumes that an
airdryerisinuse.Thetroubleshootingpresentedwillcover
notonlythe compressor itself, but also othercharging sys-
tem devices as they relate to the compressor.
WARNING! PLEASE READ AND FOLLOW
THESE INSTRUCTIONS TO AVOID
PERSONAL INJURY OR DEATH:
When working on or around a vehicle, the following
general precautions should be observed at all times.
1. Park the vehicle on a level surface, apply the
parking brakes, and always block the wheels.
Always wear safety glasses.
2. Stop the engine and remove ignition key when
working under or around the vehicle. When
working in the engine compartment, the engine
should be shut off and the ignition key should be
removed. Where circumstances require that the
enginebeinoperation,EXTREMECAUTIONshould
be used to prevent personal injury resulting from
contact with moving, rotating, leaking, heated or
electrically charged components.
3. Do not attempt to install, remove, disassemble or
assemble a component until you have read and
thoroughly understand the recommended
procedures. Useonlythepropertoolsandobserve
all precautions pertaining to use of those tools.
4. If the work is being performed on the vehicle’s air
brake system, or any auxiliary pressurized air
systems, make certain to drain the air pressure
from all reservoirs before beginning ANY work on
the vehicle. If the vehicle is equipped with an
AD-IS™airdryersystemoradryerreservoirmodule,
be sure to drain the purge reservoir.
5. Following the vehicle manufacturer’s
recommendedprocedures,deactivatetheelectrical
systeminamannerthatsafelyremovesallelectrical
power from the vehicle.
6. Never exceed manufacturer’s recommended
pressures.

15
7. Never connect or disconnect a hose or line
containing pressure; it may whip. Never remove a
component or plug unless you are certain all
system pressure has been depleted.
8. Use only genuine Bendix®replacement parts,
components and kits. Replacement hardware,
tubing, hose, fittings, etc. must be of equivalent
size, type and strength as original equipment and
bedesignedspecificallyforsuchapplicationsand
systems.
9. Components with stripped threads or damaged
parts should be replaced rather than repaired. Do
notattemptrepairsrequiringmachiningorwelding
unless specifically stated and approved by the
vehicle and component manufacturer.
10. Prior to returning the vehicle to service, make
certainallcomponentsandsystemsarerestoredto
their proper operating condition.
11. Forvehicleswith Antilock TractionControl(ATC),
theATCfunctionmustbedisabled(ATCindication
lampshouldbeON)priortoperforminganyvehicle
maintenance where one or more wheels on a
drive axle are lifted off the ground and moving.

*This guide is only for vehicles that use desiccant air dryers.
The guide consists of an introduction to air brake charging system
components, a table showing recommended vehicle maintenance
schedules, and a troubleshooting symptom and remedy section with tests
to diagnose most charging system problems.
Advanced Troubleshooting Guide
for Air Brake Compressors*
This troubleshooting guide obsoletes and supersedes all previous published
troubleshooting information relative to Bendix air compressors.
Air brake charging system:
Slow build (9.0) . . . . . . . . . . . . . . . . . 24 - 25
Doesn’t build air (10.0) . . . . . . . . . . . . . . . 26
Air dryer:
Doesn’t purge (14.0) . . . . . . . . . . . . . . . . . 27
Safety valve releases air (12.0). . . . . . . . . 27
Compressor:
Constantly cycles (15.0) . . . . . . . . . . . . . . 27
Leaks air (16.0) . . . . . . . . . . . . . . . . . . . . . 28
Safety valve releases air (11.0). . . . . . . . . 26
Noisy (18.0) . . . . . . . . . . . . . . . . . . . . . . . . 28
Reservoir:
Safety valve releases air (13.0). . . . . . . . . 27
INDEX
Air Coolant
Engine
Oil
Compressor leaks coolant (17.0) . . . . . . . . . . 28
Oil consumption (6.0) . . . . . . . . . . . . . . . . . . . 24
Oil Test Card results (1.0) . . . . . . . . . . . . . . . . 19
Oil is present:
On the outside of the compressor (2.0). . . 20
At the air dryer purge/exhaust
or surrounding area (3.0) . . . . . . . . . . . 20
In the supply reservoir (4.0). . . . . . . . 21 - 23
At the valves (5.0) . . . . . . . . . . . . . . . . . . . 23
At air dryer cartridge (7.0) . . . . . . . . . . . . . 24
In the ping tank or compressor
discharge aftercooler (8.0) . . . . . . . . . . 24
Symptom Page Number
(1) Oil Leakage at Head Gasket . . .29
(2) System Leakage . . . . . . . . . . . .29
(3) Compressor Discharge and
Air Dryer Inlet Temperature . . . .29
(4) Governor Malfunction . . . . . . . .30
(5) Governor Control Line . . . . . . . .30
(6) Compressor Unloader . . . . . . . .30
BASIC Test Information . . . . . . 32-34
Test Procedures
Maintenance Schedule and
Usage Guidelines (Table A) . . 18
Symptom Page Number
Maintenance & Usage Guidelines
16

17
Introduction to the Air Brake Charging System
Powered by the vehicle engine, the air compressor
builds the air pressure for the air brake system. The
aircompressoristypicallycooledbytheenginecoolant
system and lubricated by the engine oil supply.
The compressor's unloader mechanism and governor
(along with a synchro valve for the Bendix®DuraFlo™
596 air compressor) control the brake system air
pressure between a preset maximum and minimum
pressurelevelbymonitoringthepressureintheservice
(or“supply”)reservoir. Whentheairpressurebecomes
greater than that of the preset “cut-out”, the governor
controls the unloader mechanism of the compressor
to stop the compressor from building air and also
causes the air dryer to purge. As the service reservoir
airpressuredropsto the “cut-in” setting of the governor,
the governor returns the compressor back to building
air and the air dryer to air drying mode.
As the atmospheric air is compressed, all the water
vapor originally in the air is carried along into the air
system, as well as a small amount of the lubricating oil
as vapor.
The duty cycle is the ratio of time the compressor
spends building air to the total engine running time.
Aircompressorsaredesignedtobuildair(run“loaded”)
up to 25% of the time. Higher duty cycles cause
conditions that affect air brake charging system
performance which may require additional
maintenance. Factors that add to the duty cycle are:
air suspension, additional air accessories, use of an
undersized compressor, frequent stops, excessive
leakage from fittings, connections, lines, chambers or
valves, etc.
The discharge line allows the air, water-vapor and
oil-vapor mixture to cool between the compressor and
air dryer. The typical size of a vehicle's discharge line,
(see column 2 of Table A on page 18) assumes a
compressor with a normal (less than 25%) duty cycle,
operating in a temperate climate. See Bendix and/or
other air dryer manufacturer guidelines as needed.
Whenthetemperature ofthecompressedairthatenters
the air dryer is within the normal range, the air dryer can
remove most of the charging system oil. If the
temperature of the compressed air is above the normal
range, oil as oil-vapor is able to pass through the air
dryerandintothe airsystem. Largerdiameterdischarge
linesand/orlongerdischargelinelengthscanhelpreduce
the temperature.
The discharge line must maintain a constant slope
down from the compressor to the air dryer inlet fitting
to avoid low points where ice may form and block the
flow. If, instead, ice blockages occur at the air dryer
inlet, insulation may be added here, or if the inlet fitting
is a typical 90 degree fitting, it may be changed to a
straight or 45 degree fitting. For more information on
how to help prevent discharge line freeze-ups, see
Bendix Bulletins TCH-08-21 and TCH-08-22 (see
pages 35-37). Shorter discharge line lengths or
insulation may be required in cold climates.
The air dryer contains a filter that collects oil droplets,
and a desiccant bed that removes almost all of the
remaining water vapor. The compressed air is then
passed to the air brake service (supply) reservoir. The
oil droplets and the water collected are automatically
purgedwhenthegovernor reaches its“cut-out”setting.
Forvehicleswithaccessoriesthataresensitivetosmall
amounts of oil, we recommended installation of a
Bendix®PuraGuard®system filter,designedtominimize
the amount of oil present.
Air Dryer
Reservoir Drain
Service Reservoir
(Supply Reservoir)
Compressor
Governor
(Governor plus Synchro valve
for the Bendix®DuraFlo™596™
Compressor)
Discharge
Line Optional “Ping” Tank
Optional Bendix®PuraGuard®
System Filter or PuraGuard®
QC™Oil Coalescing Filter
The Air Brake Charging System supplies the
compressed air for the braking system as well as other
air accessories for the vehicle. The system usually
consists of an air compressor, governor, discharge line,
air dryer, and service reservoir.

18
Compressor with up to 25% duty cycle
Footnotes:
1 With increased air demand the air dryer cartridge needs to be replaced more often.
2 Use the drain valves to slowly drain all reservoirs to zero psi.
3 Allow the oil/water mixture to fully settle before measuring oil quantity.
4 Tocounterabovenormaltemperaturesattheairdryerinlet,(andresultantoil-vaporpassing
upstream in the air system) replace the discharge line with one of a larger diameter and/
or longer length. This helps reduce the air's temperature. If sufficient cooling occurs, the
oil-vaporcondensesandcanberemovedbytheair dryer. Dischargeline upgrades are not
coveredunder warranty. Note:Tohelppreventdischargelinefreeze-ups,shorterdischarge
linelengthsor insulationmayberequiredincoldclimates. (See Bendix BulletinsTCH-08-21
and TCH-08-22, included inAppendix B, for more information.)
5 For certain vehicles/applications, where turbo-charged inlet air is used, a smaller size
compressor may be permissible.
Recom- Recom- Acceptable
Typical Discharge mended mended Reservoir
Compressors Line Air Dryer Reservoir Oil Contents3
No. of Spec'd Cartridge Drain at Regular
Axles Replacement1Schedule2Drain Interval
High Air Use
Low Air Use
e.g. Double/triple trailer, open
highway coach/RV, (most)
pick-up & delivery, yard or
terminal jockey, off-highway,
construction, loggers, concrete
mixer, dump truck, fire truck.
e.g. Line haul single trailer
w/o air suspension, air over
hydraulic brakes.
e.g. Line haul single trailer
with air suspension,
school bus.
e.g. City transit bus, refuse,
bulk unloaders, low boys,
urban region coach, central
tire inflation.
5
or
less
5
or
less
8
or
less
12
or
less
Table A: Maintenance Schedule and Usage Guidelines
Recom-
mended
Every
Month -
Max of
every 90
days
Every
Month
Every 3
Years
Every 2
Years
Every
Year
I.D.
Vehicle Used for:
Column 1 Column 2 Column 3 Column 4 Column 5
Regularly scheduled maintenance is the single most important factor in maintaining the air brake charging system.
Note: Compressor and/or air dryer
upgradesare recommendedin cases
where duty cycle is greater than the
normal range (for the examples
above).
For Bendix®Tu-Flo®550 and 750
compressors, unloader service is
recommended every 250,000 miles.
Length
6 ft.
1/2 in.
9 ft.1/2 in.
12 ft.
1/2 in.
5/8 in. 12 ft.
BASIC test
acceptable
range:
5 oil units
per month.
See
appendix
A.
For oil carry-over
control4 suggested
upgrades:
3/4 in. 15 ft.
For oil carry-over
control4 suggested
upgrades:
5/8 in. 15 ft.
For oil carry-over
control4 suggested
upgrades:
5/8 in. 9 ft.
For oil carry-over
control4 suggested
upgrades:
5/8 in. 12 ft.
Compressor with less than 15% duty
cycle
Compressor with up to 25% duty cycle
Compressor with up to 25% duty cycle
Bendix®BA-921™air compressor
Bendix®Tu-Flo®550 air compressor
Bendix®Tu-Flo®750 air compressor
Bendix®BA-922™, or DuraFlo™596air compressor
BASIC test
acceptable
range:
3 oil units
per month.
See
appendix
A.
For the
BASIC
Test Kit:
Order
Bendix
P/N
5013711

19
ü
Air Brake Charging System Troubleshooting
How to use this guide:
1.0 Oil Test Card
Results Not a valid test. Discontinue using this test.
Do not use this card test to diagnose
compressor "oil passing" issues. They are
subjective and error prone. Use only the
BendixAir System InspectionCup(BASIC)test
and the methods described in this guide for
advanced troubleshooting.
The Bendix®BASIC test should be the
definitive method for judging excessive oil
fouling/oil passing. (See Appendix A, on
page 32 for a flowchart and expanded
explanation of the checklist used when
conducting the BASIC test.)
Symptom: What it may indicate: What you should do:
Find the symptom(s) that you see, then move to the right to
find the possible causes (“What it may indicate”) and
remedies (“What you should do”).
Review the warranty policy before performing any intrusive
compressor maintenance. Unloader or cylinder head gasket
replacement and resealing of the bottom cover plate are
usually permitted under warranty. Follow all standard safety
procedures when performing any maintenance.
Look for:
Normal - Charging system is working within
normal range.
Check - Charging system needs further
investigation.
û
Bendix®
BASIC Test
WARNING! Please READ and follow these instructions to
avoid personal injury or death:
When working on or around a vehicle, the following general
precautions should be observed at all times.
1. Park the vehicle on a level surface, apply the parking
brakes, and always block the wheels. Always wear safety
glasses.
2. Stop the engine and remove ignition key when working
under or around the vehicle. When working in the engine
compartment,theengine should beshutoffandtheignition
keyshould beremoved.Where circumstancesrequirethat
the engine be in operation, EXTREME CAUTION should
be used to prevent personal injury resulting from contact
withmoving,rotating,leaking,heated or electricallycharged
components.
3. Donot attempt toinstall,remove,disassemble or assemble
a component until you have read and thoroughly
understand the recommended procedures. Use only the
proper tools and observe all precautions pertaining to use
of those tools.
4. If the work is being performed on the vehicle’s air brake
system, or any auxiliary pressurized air systems, make
certain to drain the air pressure from all reservoirs before
beginning ANY work on the vehicle. If the vehicle is
equipped with an AD-IS™air dryer system or a dryer
reservoir module, be sure to drain the purge reservoir.
5. Following the vehicle manufacturer’s recommended
procedures, deactivate the electrical system in a manner
that safely removes all electrical power from the vehicle.
6. Never exceed manufacturer’s recommended pressures.
7. Never connect or disconnect a hose or line containing
pressure;it maywhip. Never removea componentorplug
unless you are certain all system pressure has been
depleted.
8. Useonly genuine Bendix®replacementparts,components
andkits.Replacement hardware, tubing,hose,fittings, etc.
must be of equivalent size, type and strength as original
equipment and be designed specifically for such
applications and systems.
9. Components with stripped threads or damaged parts
should be replaced rather than repaired. Do not attempt
repairs requiring machining or welding unless specifically
stated and approved by the vehicle and component
manufacturer.
10.Prior to returning the vehicle to service, make certain all
components and systems are restored to their proper
operating condition.
11. For vehicles with AntilockTraction Control (ATC), the ATC
function must be disabled (ATC indicator lamp should be
ON) prior to performing any vehicle maintenance where
oneor more wheelson a driveaxle are liftedofftheground
and moving.

Symptom: What it may indicate: What you should do:
20
2.2 Oil leaking
from compressor: (a)Excessive leak at head gasket.
(b)Leak at bottom cover plate.
(c)Leak at internal rear flange gasket.
(d)Leak through crankcase.
(e)(If unable to tell source of leak.)
ðGo to Test 1 on page 29.
ðReseal bottom cover plate using RTV
silicone sealant.
ðReplace compressor.
ðReplace compressor.
ðClean compressor and check periodically.
Air brake charging system functioning
normally.
ðAir dryers remove water and oil from the
air brake charging system.
Check that regular maintenance is being
performed. Return the vehicle to service.
An optional kit (Bendix piece number
5011327 for the Bendix®AD-IS™or AD-IP™
air dryers, or 5003838 for the Bendix®
AD-9™air dryer) is available to redirect the
air dryer exhaust.
3.0 Oil at air dryer
purge/exhaust or
surrounding area
(c)
2.0 Oil on the
Outside of the
Compressor
Find the source and repair. Return the vehicle
to service.
ðRepair or replace as necessary. If the
mounting bolt torques are low, replace the
gasket.
ðReplace the fitting gasket. Inspect inlet
hose and replace as necessary.
ðReplace gasket or fitting as necessary to
ensure good seal.
ðInspect and repair as necessary.
Engine and/or other accessories
leaking onto compressor.
(a)Leak at the front or rear (fuel
pump, etc.) mounting flange.
(b)Leak at air inlet fitting.
(c)Leak at air discharge fitting.
(d)Loose/broken oil line fittings.
2.1 Oil leaking at
compressor / engine
connections:
(a)
ð
ð
Head
gasket
and rear
flange
gasket
locations.
(c)