TL;DR
Pressure loss under load means fluid is bypassing without doing work. Start with the relief valve. If it is set correctly and not stuck, measure pump case drain flow to distinguish a pump bypass fault from an actuator bypass fault.
What you might see
- system pressure below setpoint under load
- actuators weak or unable to hold position
- pump running continuously without building pressure
- relief valve vent audible at normal operating load
Likely causes
Relief valve stuck open or set lower than the required system pressure
Worn PV pump with excessive internal bypass reducing effective output pressure
Internal cylinder bypass past worn piston seals, preventing pressure buildup
Cracked or failed hydraulic hose internally (inner tube collapsed, restricting flow and reducing pressure downstream)
Required tools
- Pressure gauge calibrated to system range
- LOTO kit
- Inline flowmeter (for case drain test)
- Graduated container and stopwatch
Safety first
- Never adjust a relief valve without a pressure gauge in the circuit. Over-pressurizing the system can burst hoses or fittings.
- Depressurize the system before breaking any hydraulic connection for gauge installation.
Procedure
- 1
Connect a pressure gauge directly at the pump outlet port with a tee. Compare the reading to the relief valve setting. If pump outlet equals relief valve setting, the pump is producing pressure.[1]
- 2
Lock all actuators and re-measure pump outlet pressure. If pressure now reaches the relief setting, the loss is in an actuator or directional valve bypass, not the pump.
- 3
Locate the main relief valve. Measure its setting against the system design pressure. If set too low, adjust using the relief valve adjustment screw per the Parker specification.[1]
Warning: Do not stand in front of a relief valve adjustment port when adjusting under pressure. Use a pressure gauge to monitor and stop at the design pressure. - 4
Measure pump case drain flow (see the slow-operation procedure on this machine). Case drain above 10% of rated displacement confirms pump internal bypass.
- 5
If the pump tests healthy, identify which actuator or valve is bypassing by isolating circuits one at a time and watching for pressure recovery at the pump outlet.
- 6
Inspect hydraulic hoses for kinking, external damage, or age-related inner tube collapse. Replace any hose that is soft, bulging, or more than 5 years old.
Sources
Parker PV Series Variable Volume Piston Pump Service Manual, Parker Hannifin Corporation
Parker PV Series Piston Pump Service Manual, system pressure testing and relief valve adjustment (general)
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