TL;DR

Low hydraulic pressure is most often a worn main pump, drifted relief valve, or leaking line. Measure standby and relief pressure before pulling the pump.

What you might see

slow or sluggish boom and stick
system pressure below rated spec
hydraulic oil temperature rising fast
pump noise louder than normal

Likely causes

Tap to mark as ruled out

Main hydraulic pump worn, reduced volumetric efficiency from piston/swash plate wear
Main system relief valve set too low or sticking open
Internal leakage in a hydraulic cylinder or motor worn past tolerances
Low hydraulic oil level or severely contaminated oil thinning film strength

Required tools

Calibrated hydraulic pressure gauge with appropriate test-point adapters
Hydraulic oil sample kit
IR thermometer
LOTO kit

Safety first

Engage the hydraulic lockout lever before leaving the cab or working near any actuator. Unsupported implements can fall.
Hydraulic oil under high pressure can inject through skin. Never check for leaks with bare hands.
Falling material hazard: keep clear of the bucket and boom during all tests.

Procedure

9 steps · ~60 min

1
Park the shovel on level ground, lower the bucket to the ground, and shut down the engine.
Warning: Engage the hydraulic lockout lever in the cab before leaving the operator station.
2
Check the hydraulic oil level in the reservoir sight glass. Top up if low with the specified grade.
3
Connect a calibrated hydraulic pressure gauge to the test point on the main pump outlet.[1]
4
Start the engine and warm the hydraulic oil to operating temperature (above 50 degC).
5
Read standby pressure with controls at neutral, then activate a function against the relief and read relief pressure. Compare both values to the Caterpillar specification for this machine.[1]
6
If standby pressure is in spec but relief pressure is low, adjust or replace the main system relief valve.
7
If both pressures are low, take a hydraulic oil sample for particle count and viscosity analysis. Pump wear produces metallic fines.
8
Inspect all visible hydraulic lines and fittings for external leaks. Wet dust on lines near fittings indicates a seeping joint.
9
If pump wear is confirmed by low efficiency and metallic contamination, schedule a pump overhaul at the next planned maintenance stop.

Sources

Caterpillar CAT 6060 / 6090 Hydraulic Mining Shovel / Excavator general technical documentation, Caterpillar
Caterpillar hydraulic shovel service and maintenance documentation, hydraulic pressure test procedures (general)

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