Fault Phenomenon: CAT rotary drilling rig after the mast is raised and the engine is off or other actions are performed, the mast slowly lowers automatically.

Troubleshooting Process:
– Checked cylinder: luffing 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 luffing cylinder head end is stuck or worn, unable to seal the high-pressure oil.

Required Parts: Clean or replace holding valve assembly.