BENDIX LQ-5 BOBTAIL RATIO VALVE User manual

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®
SD-03-953
Bendix®LQ-5™Bobtail Ratio Valve
Although it is not mandatory, the LQ-5™valve is used in
conjunctionwiththeBP-R1™bobtailbrakeproportioningrelay
valve,onlywhenacustomerspecifiesafrontaxleratiovalve.
The BP-R1™valve and LQ-5™valve combine to reduce
stoppingdistanceandimprovetractorcontrolinbobtailmode.
(FormoreinformationontheBP-R1™valve,seeSD-03-1067.)
OPERATION
GENERAL
Afteraninitialdelivery,theLQ-5™valvelimitsfrontaxlebrake
pressureduring normaltractor-trailer operation. TheLQ-5™
valve has two spring-loaded pistons: a blend back piston
andaproportioningpiston.The“stepped”proportioningpiston
determinesactualair pressurereductionand theblendback
piston allows the valve’s delivery pressure to “blend back”
andreacha1:1ratioduringhighpressurebrakeapplications.
DESCRIPTION
TheLQ-5™bobtail ratio valveisused on thefront (steering)
axle of tractor air brake systems to reduce brake applica-
tion pressure during normal tractor-trailer operation. It is
designedfor tractor systemsonly and replacestheexisting
frontaxlelimitingvalve.Itisnottobeusedon straighttrucks
or towing trucks. Also, a quick release valve must be in-
stalled between the LQ-5™valve delivery and the brake
chambers.
IMPORTANT: Do not replace the BP-1™front brake pro-
portioning valve with the LQ-5™valve. The LQ-5™valve
will not function properly in the BP-1™valve system.
Caution:TheLQ-5™valvecan replacetheLQ-4™ratiovalve
ontractors.The LQ-5™valvecontrolport mustbeconnected
to the trailer supply line. When the tractor is disconnected
fromthetrailer,bobtailtractorbrakingperformanceisimproved
because full brake valve pressure will be applied to the
steeringaxle brakes.
FIGURE 1 - LQ-5™BOBTAIL RATIO VALVE
3/8 P.T.
SUPPLY PORT
1/8 P.T.
CONTROL PORT
1.5" .340”
MOUNTING
HOLES (2)
STAMPED LQ-5
REPLACEMENT
PC. NO.
3/8 P.T.
DELIVERY PORT

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TRACTOR-TRAILER MODE
Brake Application—LQ-5™Valve Limiting
Duringtractor-trailer operation, thetrailersupply valve (red
octagonal button) is depressed and delivers air pressure
simultaneouslyto the trailer and to theLQ-5™valvecontrol
port.Air pressure enters the LQ-5™valve control port and
displaces the blend back piston, against the resistance of
its spring, until the piston contacts the cover.
Control pressure holds the blend back piston against the
coverandthe“initial delivery”springforces theproportioning
piston to its seat in the body, which opens the inlet. The
“initialdelivery”pressureisdesigned toensurethatthebrake
liningscontactthe brakedrumsbefore proportioningoccurs.
Theexhaustvalve issealeddue toitsspring. Brakevalveair
pressure enters the supply port, moves through the center
ofthe pistons,around theinlet/exhaust valve,and outto the
frontaxle actuators.
Air pressure being delivered to the brake actuators is also
presentbeneaththeproportioningpiston. Becausethe area
onthe deliveryside ofthe proportioningpiston islarger than
that on the supply side, the delivery side needs less air
pressure to apply the same force.
Thereforeafter theinitial deliverypressure, theLQ-5™valve
delivers less air pressure to the front axle than it receives
fromthebrake valve.
The LQ-5™valve also allows full service pressure to pass
throughitwhenthetrailersupplyvalve(redoctagonalbutton)
is pulled out—when the tractor is running in bobtail mode.
Note:LQ-5™valve controlpressure,which comes fromthe
trailer supply line, will vary. As a result, the LQ-5™valve
delivery and blend back ranges will also vary, because the
blendback piston works against controlpressure. Figure 3
shows the LQ-5™valve delivery range at two control
pressures: 90 psi and 120 psi.
FIGURE 2 - LQ-5™BOBTAIL RATIO VALVE INSTALLATION
FIGURE 3 - LQ-5™BOBTAIL RATIO VALVE PERFORMANCE
CURVE
CONTROL PRESSURE
120 PSI
NO CONTROL
PRESSURE
0BRAKE VALVE DELIVERY
PRESSURE
CONTROL
PRESSURE
90 PSI
FRONT
AXLE
BRAKE
CHAMBER
PRESSURE
INITIAL
DELIVERY
PRESSURE
PP-7™
TRAILER
SUPPLY
VALVE
SUPPLY
LQ-5™
VALVE
QUICK RELEASE
VALVE
BRAKE
CHAMBER
BRAKE CHAMBER FRONT AXLE
SERVICE RESERVOIR
GAUGE
LP-3™LOW
PRESSURE
INDICATOR
BRAKE
VALVE
DELIVERY

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BALANCEDBRAKEVALVEAPPLICATION-LQ-5™
VALVE HOLDING
Theforcesactingonthe proportioningpistonareequalwhen
airpressure beneaththe pistonis approximatelyone-half of
that above. (For example, if a 40 psi brake application is
made, approximately 20 psi would be present beneath the
proportioningpiston.)At this point, theproportioningpiston
lifts and closes the inlet. The exhaust passage remains
sealed. With both the inlet and exhaust valves closed, the
LQ-5™valveisbalanced—front axle air pressure is neither
increasingnor decreasing.
BRAKEVALVE RELEASE
Uponbrakevalverelease,airpressure abovetheblendback
pistonreturns tothe brakevalve andexhausts.Airpressure
fromthe delivery lineliftsthe proportioning pistonand inlet/
exhaust valve away from the exhaust seat in the body. Air
then flows out the open exhaust passage. Air from the
actuators exhausts through the quick release valve. (See
Figure2.)
Ifthe service brakes arerapidly released, air pressuremay
liftthe inlet/exhaust valve further againsttherelatively light
resistanceofthe valvespring.Air wouldthen alsoflowback
throughthe open inlet andoutto the brake valve.
FIGURE 5 - LQ-5™BOBTAIL RATIO VALVE HOLDING:
TRACTOR-TRAILER MODE
FIGURE 4 - LQ-5™BOBTAIL RATIO VALVE LIMITING: TRACTOR-TRAILER MODE (SHOWN DURING INITIAL DELIVERY)
INLET/
EXHAUST
VALVE
COVER
BODY
PROPORTIONING
PISTON
INITIAL
DELIVERY
PISTON
BLEND
BACK
PISTON
BLEND
BACK
SPRING
DUAL
BRAKE
VALVE
TRAILER
SUPPLY
VALVE
FRONT
BRAKE
CHAMBER
APPROXIMATELY 1/2 OF
BRAKE VALVE
APPLICATION PRESSURE
BRAKE VALVE APPLICATION
PRESSURE

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FIGURE 8 - LQ-5™BOBTAIL RATIO VALVE MODE
FIGURE 7 - LQ-5™BOBTAIL RATIO VALVE BLENDING
BACK: TRACTOR-TRAILER MODE
BRAKEAPPLICATION—LQ-5™VALVE
BLENDING BACK
Servicebrake pressure enters theLQ-5™valvesupply port
and combines with the force of the blend back spring to
work against control pressure. When service pressure is
sufficiently high (see Figure 3), the total downward force
beginsto overcomethe upwardforce ofcontrol pressureon
the blend back piston. The blend back piston is forced into
contactwiththeproportioningpiston.Both pistonsnowmove
as one. Because of the added surface area of the blend
backpiston, LQ-5™valve deliverypressure“catches up” as
supply pressure increases. When the piston assembly
contacts its seat in the body, the LQ-5™valve delivers full
application pressure. There is then a 1:1 ratio between
supplyand delivery.
BOBTAIL MODE
BRAKE VALVE APPLICATION, BALANCE,
RELEASE
When the trailer supply valve is pulled out, air in the LQ-5™
valve control port flows back to the trailer supply valve and
exhauststoatmosphere.With nocontrolpressure,theblend
backpistonisforcedbyits springtocontacttheproportioning
piston.Theinlet/exhaust valveisheld byitsspring andseals
the exhaust passage. Both pistons, now moving as one,
reach their stop in the body.
Thisisthepositionforbobtailmodeapply, holdand release.
TheLQ-5™valvedeliversfullservice applicationpressure in
FIGURE 6 - LQ-5™BOBTAIL RATIO VALVE RELEASING:
TRACTOR-TRAILER MODE
Whenairpressurebeneaththeproportioningpistondepletes
to less than approximately 8 psi, the valve spring is able to
seat the exhaust valve. The “initial delivery” spring keeps
theinletopen andallowsair fromtheLQ-5™valve deliveryto
flowthrough thevalve, out thesupply andback to thebrake
valve.

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bobtailmode.Uponbrakevalverelease,deliveryairpressure
flowsback through the LQ-5™valve,to thebrakevalve and
outto atmosphere.Airfrom the actuatorsexhauststhrough
thequick releasevalve.
WARNING! PLEASE READ AND FOLLOW
THESE INSTRUCTIONS TO AVOID
PERSONAL INJURYOR 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
enginebe in operation, EXTREME CAUTIONshould
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. Useonly theproper 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,makecertain to drain theairpressure 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
recommendedprocedures, deactivate theelectrical
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 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
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
certainallcomponentsandsystemsarerestoredto
their proper operating condition.
PREVENTIVE MAINTENANCE
GENERAL
Performthefollowingtests andinspectionsat theprescribed
intervals.If theLQ-5™valvefails to functionas described or
if leakage is excessive, repair the valve or replace it with a
neworgenuineBendixremanufactured unit,availableatany
authorizedparts outlet.
Important: Review the Bendix Warranty Policy before
performinganyintrusivemaintenanceprocedures.Awarranty
maybe voidedif intrusive maintenanceis performedduring
thewarranty period.
No two vehicles operate under identical conditions, as a
result, maintenance intervals may vary. Experience is a
valuableguideindeterminingthe bestmaintenance interval
for air brake system components.At a minimum, the valve
shouldbeinspectedevery6monthsor1500operatinghours,
whichevercomesfirst,forproperoperation.Shouldthevalve
notmeet the elements of theoperationaltests noted in this
document,furtherinvestigation andserviceof thevalvemay
berequired.
OPERATION & LEAKAGE TESTS
OPERATION TEST
ToproperlytesttheLQ-5™valve, usea pairoftest gaugesor
gaugesof known accuracy.
1. With the trailer supply valve pushed in (tractor-trailer
mode), install two air gauges in the front axle service
system: one between the brake valve and the LQ-5™
valveand one atafront axle actuator.
2 Buildsystem pressure to governor cut-out setting.
3. Makea servicebrake applicationwith thefoot valveand
have someone observe the gauges. When the gauge
betweenthe footvalveand theLQ-5™valvereaches 40
psi, the actuator gauge should read approximately 20
psi (±5 psi).
4. Makeafullservicebrakeapplication.Bothgaugesshould
readthe same pressure.
5. With the trailer supply valve pulled out (bobtail mode),
makea30-40psiservicebrakeapplication.Bothgauges
shouldread the same pressure.
6. Removethe test gauges.
LEAKAGE TEST
1. Buildsystempressuretogovernorcut-out.Withthetrailer
supplyvalvepushedin(tractor-trailermode),applyasoap
solution to the LQ-5™valve exhaust port. The leakage
should not exceed a 1" bubble in less than 3 seconds.

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2. Makeand holdafull brakeapplication and applya soap
solutionto theexhaust portand aroundthe coverwhere
it joins the body. The leakage should not exceed a 1"
bubble in less than 3 seconds.
VALVE REMOVAL
1. Secure the vehicle on a level surface by means other
than the brakes.
2. Drainair system completely, makingsureall reservoirs
areat atmospheric pressure.
3. Identify and mark all air lines and their respective con-
nectionson thevalve.
4. Disconnect all air lines.
5. Removethevalvefromthevehicle.
VALVE INSTALLATION
Ifthe vehicle has afrontaxle ratio or limitingvalve (such as
theBendix® LQ-4™valve),it canbereplaced with an LQ-5™
valve. The LQ-5™valve control port must be connected to
the trailer supply line.Also, a quick release valve must be
installed between the LQ-5™valve delivery and the brake
chambers.
1. Install all air line fittings making certain thread sealing
materialdoes notenterthe valve.
2. Mount the assembled valve on the vehicle with the
exhaustpointing down.
3. Reconnect all air lines to the valve using the identifica-
tion madeduring Valve Removal,Step 3.
4. After installing the LQ-5™valve, test all air fittings for
excessiveleakage and tightenasneeded.
DISASSEMBLY
The following disassembly and assembly procedures are
forreferenceonly.Alwayshave theappropriatemaintenance
kitonhand anduseitsinstructionsin lieuofthosepresented
here.Refer to Figure9 throughout theprocedures.
Caution:TheLQ-5™valvemaybelightlyclampedinabench
vise, but overclamping will damage the valve and cause
leakageand/or malfunction. Ifa vise is used,clamp around
thesupplyportand the mounting surface.
1. Removeall airfittings fromthevalve.
2. Before removing the four Phillips head screws that
secure the cover to the body, mark the relationship
betweencoverandbody.
3. Remove and retain the screws and the metal tag with
theLQ-5™valvepiecenumberonit.Separatecover from
body. Removeanddiscard o-ring(1).
4. Removethe blendback and proportioning pistonsfrom
the body.
5. Separateand retainthe pistons. Removeand retainthe
springseat from the proportioningpiston.
6. With a pair of needle nose pliers, grasp the end of the
valvespring(6)insidethe proportioningpiston. Remove
thespring by pulling andturning clockwise. Discard the
spring.
7. Removeanddiscardinlet/exhaustvalve(5)frompropor-
tioningpiston.
8. Remove and discard o-rings (2), (3), (4) from the
pistons.
CLEANING & INSPECTION
1. Clean all metal parts with mineral spirits or an equiva-
lentsolvent.
2. Thoroughlydry all metal parts.
3. Inspectinterior andexterior of allmetal parts thatwill be
reused for severe corrosion, pitting or cracks. Superfi-
cial corrosion and/or pitting on the exterior of the body
orcover is acceptable.Replace entire valveif interior of
bodyorcovershowssignsofcorrosionorpitting.
4. Inspect body and cover bores for deep scuffing or
gouges.Replace entirevalveif eitheris found.
FIGURE 9 - LQ-5™BOBTAIL RATIO VALVE
3
INITIAL
DELIVERY
SPRING
1
2
BLEND
BACK
PISTON
4
BODY
5
6
SPRING
SEAT
COVER
BLEND
BACK
SPRING
PROPORTIONING
SPRING

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5. Make certain all air channels are clear and free of
obstruction.
6. Inspect all pipe threads and make sure they are clean
andfree ofthreadsealant.
7. Inspect all air line fittings for corrosion and replace as
necessary.
ASSEMBLY
1. Before assembly, lubricate o-rings, pistons, and body
andcover boreswith the lubricantin theBendixmainte-
nance kit.
2. Install o-rings (3 & 4) on proportioning piston. Install
o-rings(2 & 4)on blend backpiston. Install o-ring(1) on
cover.
3. Place inlet/exhaust valve (5) into the inner diameter of
theproportioning pistonand retainwithvalve spring(6).
Useneedlenosepliers to install the spring by grasping
it on the bar that intersects its end and turning clock-
wise while pressing down. The bar must be away from
(nottouching)thevalve.
Note: Makesure spring coils donot overlap each other
asthe spring compressesintothe proportioning piston.
Also, make sure the valve is square, contacts the seat,
anddoesn’t bind.
4. Insertthe proportioning pistonintothe small boreofthe
blendback piston and installthe piston assembly inthe
body.
5. Install spring seat into the piston assembly bore, small
endfirst.
6. Installthe “initial delivery”springon the innerwall of the
springseat.
7. Installthe blend backspring in the innerdiameter of the
blend back piston so that it surrounds the “initial
delivery”spring.
8. Place the cover on the body in the proper position, as
marked in Disassembly, Step 2. Make sure the springs
are square when the cover is attached to the body.
9. Usingthe Phillips head screws and their lock-washers,
secure the cover to the body.Torque the screws to 50-
80inch pounds.
10. Install all air line fittings, making sure thread sealing
materialdoes notenterthe valve.
11. MountLQ-5™valveonthevehicleandperform Operation
and Leakage Tests before returning the vehicle to
service.

8BW1625 © 2004 Bendix Commercial Vehicle Systems LLC All rights reserved. 4/2004 Printed in U.S.A.