Hydraulic system losing pressure under load after warm-up

Troubleshooting & Failures
1

Problem:
Hydraulic system loses pressure under load after running for ~20–30 minutes.

System:
Industrial hydraulic setup with fixed displacement pump, pressure relief valve, and directional control valves. Recently replaced pump.

Expected behavior:
System should maintain stable pressure under load during continuous operation.

Actual behavior:
Pressure is stable during startup (cold condition), but gradually drops after warm-up. Load performance decreases significantly. No visible external leaks.

What I tried:

  • Checked for external leaks → none found
  • Replaced hydraulic pump → no improvement
  • Verified pressure relief valve setting
  • Checked oil level and condition

Error / Logs:
No error codes or alarms from control system. Manual pressure readings show drop after temperature increase.

2

We had a similar issue before.

If the pressure drops only after warm-up, I would strongly suspect internal leakage, either in the pump (even if new) or more likely in a valve block.

Thermal expansion can increase internal clearances, especially in worn components.

I’d suggest:

  • Checking valve block leakage
  • Monitoring oil temperature vs pressure drop
  • Verifying if pressure holds when isolated

Could be internal bypass rather than external leak.

3

Quick update:

I checked for internal leakage as suggested.

  • Isolated sections of the system → pressure still drops after warm-up
  • Monitored oil temperature → around 55–60°C when issue starts
  • Relief valve seems stable and not opening prematurely

However, I noticed that when the system is unloaded, pressure holds fine even when warm.

The drop only happens under load.

Could this still point to internal leakage in the valve block, or should I be looking at something else?

4

Final update:

Replacing the directional valve significantly improved the issue, but I still see a slight pressure drop under heavy load.

Not sure if it’s normal behavior or if there’s still minor internal leakage somewhere else.

At least the system is now usable, but I’m still not fully convinced this is the complete root cause.