Pump · High
Pressure drops under load, everything slows
Two functions slow at once and the gauge sags when the bucket bites. When several circuits fail together, the fault is upstream of all of them: relief, regulator, or the pump's rotating group.
What it usually is
| Share of cases | Cause and how to check it |
|---|---|
| 34% | Main relief valve stuck partially open Dead-head a function and read the main relief. A value 10–15% below rated and a hot tank confirm it. |
| 32% | Regulator / compensator drift Watch pump pressure at standby. If it hunts or sits low at no flow, the regulator is the suspect, not the rotating group. |
| 22% | Worn piston group losing volumetric efficiency Flow test at rated pressure: below 90% of catalogue flow means the group is tired. |
| 12% | Control valve spool / negative control Check standby pressure and the negative-control line for leaks before opening the pump. |
Field tests
1. Standby and stall
Two numbers, five minutes: standby pressure (no function) and stall pressure (dead-head). Everything else follows from the gap between them.
2. Flow, not just pressure
A pump can hold pressure and still deliver half the flow. Measure litres per minute at rated pressure.
3. Regulator response
Load and unload a function and watch the pressure recover within one second. Slow recovery is a regulator, not a pump.
What actually fixes it
A regulator can be replaced on the machine. A rotating group cannot. The quote should separate the two, because the price difference is not small.
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