A contractor reported that a Komatsu PC200-8 excavator's bucket would slowly drift open (uncurl) when carrying a load over the side. The boom and stick functions held their position correctly. The operator initially suspected a leaking bucket cylinder seal, but the cylinder rods showed no external leaks.
The technician performed a "cylinder by-pass test" by pressurizing the bucket cylinder and shutting off the engine. The drift was present, but when the control lever was moved to the "hold" position, the drift slowed slightly. This indicated the issue was in the main control valve, not the cylinder. The main control valve for the bucket circuit was disassembled. Inside, the "load check valve" was found to be stuck open.
The load check valve's purpose is to hold the pressure in the cylinder when the control valve is in neutral. A tiny piece of debris from a previously burst hose had lodged in the check valve's seat, preventing it from sealing. This allowed the high-pressure oil from the bucket cylinder to bleed back to the tank through the control valve housing. The technician cleaned the valve seat with a polishing stone and replaced the check valve spring. The system was flushed, and new filters were installed. The bucket drift was eliminated, restoring the machine's lifting safety.
