Fault Phenomenon: Komatsu PC300 excavator after the boom is lifted and the engine is off or other actions are performed, the boom slowly sinks automatically.

Troubleshooting Process:
– Checked cylinder: boom cylinder piston seals worn, internal leakage.
– Checked holding valve: cylinder head end holding valve internal leakage.
– Checked lines: no external leakage.
– Checked oil temperature: normal.

Root Cause: The hydraulic holding valve on the boom cylinder head end is stuck or worn, unable to seal the high-pressure oil.

Required Parts: Clean or replace holding valve assembly.