jack is extended very quickly under no load. In this situation, the back pressure and
the internal friction of the jack add together to produce high gauge readings
associated with the rapid movement of the jack. It is virtually nonexistent at lower
flow rates like 1-2 gpm. The gauge reading will drop to 0 when the circuit is
deactivated, the hydraulic power supply is turned off and the control valve handle is
cycled back and forth between the extend and retract positions. Be sure to follow this
procedure prior to disconnecting the hydraulic hoses. If the trapped back pressure is
not relieved prior to disconnecting the hoses, the hydraulic couplers will be difficult
and sometimes impossible to reconnect. If the gauge reading does not drop to zero
with everything turned off after cycling the valve, then the gauge is defective and
should be replaced.
TRAPPED PRESSURE
Trapped pressure can cause hydraulic couplers to become difficult and sometimes
impossible to connect by hand. This is a normal occurrence with almost all types of
hydraulic tools and couplers.
The three most common sources of trapped pressure are:
1. Failure to relieve any trapped back pressure in the tool circuit prior to
uncoupling hoses from tools. This is avoided by cycling the tool control valves
(on and off, up and down, extend and retract, etc.) several times after turning
off the hydraulic flow from the power unit but prior to uncoupling.
2. Sunlight heating hoses, tools, especially dark colored items.
3. Uncoupling the hoses prior to turning off the hydraulic flow from the power
unit.
The proper practices when switching hydraulic tools will avoid trapped pressure:
1. Turn hydraulic flow off at power unit.
2. Cycle the tool control valve off and on several times.
3. Disconnect hoses from tool #1.
4. Connect couplers to tool #2.
If trapped pressure does occur, it must be relieved. Here are some suggestions on
how:
1. Loosen a fitting very slowly and carefully to allow some fluid to “leak out”. This
will relieve the trapped pressure.
a. CAUTION: Hydraulic fluid can squirt out at very high velocities which
can cause injury. Always cover the fitting to be loosened with a rag and
always wear appropriate safety equipment especially glasses.
b. The best choice is a JIC or Flare fitting because of its ease of loosening
and tightening. If an O-ring style fitting is chosen, be sure to properly