A heavy demolition contractor brought in a Kobelco SK300LC excavator suffering from two distinct but related issues: the boom would drift down rapidly when parked with a load, and the hydraulic oil temperature would spike to 105°C after just one hour of operation. The operator also noted that the boom lacked power during heavy lifts. Initial diagnostics read no active ECM codes, suggesting the issue was purely mechanical or hydraulic.
To diagnose the boom drift, the technician performed a "holding valve" test. The boom was raised halfway, the engine shut off, and the pilot pressure was safely relieved. If the drift was caused by the boom control spool leaking, the drift would stop immediately. The boom continued to drop, confirming the issue was inside the boom hydraulic cylinders or the holding circuit.
Before pulling the heavy boom cylinders, the technician checked the main relief valve for the boom circuit. Upon removing the relief valve cartridge, the internal poppet was found stuck open with a sliver of hardened O-ring material. This caused the main hydraulic pump to constantly dump high-pressure fluid straight to the tank, generating the extreme heat and robbing the boom of its full lifting power.
However, a stuck relief valve alone wouldn't cause the boom to drop under a static load. The sliver of O-ring had to come from somewhere. The boom cylinders were removed and disassembled. Inside the rod-side cylinder, the composite piston seal had completely shattered into multiple pieces, and the backup ring was deeply scored. With the piston seal destroyed, high-pressure fluid from the bore side was bypassing the piston and pressurizing the rod side. This bypass created a closed-loop siphon effect where oil just circulated from one side of the piston to the other, causing the rapid drift. The fragmented pieces of the piston seal were the exact material that had lodged the main relief valve.
The root cause was traced back to a previous maintenance error. The wrong bore size piston seal had been installed during an earlier repair, leaving excessive clearance. Under high shock loads during demolition, the seal twisted and fractured. The repair involved installing a complete, correct-spec piston seal kit in both boom cylinders. The stuck main relief valve was cleaned, its seat lapped, and reinstalled. The entire hydraulic system was flushed with flushing oil to remove any remaining composite debris before refilling with fresh ISO 68 fluid. The result was a boom that held solidly with zero drift, and operating temperatures stabilized at a normal 75°C.
