Fault Phenomenon: Zoomlion ZLJ5430THB boom is clean when cold. During high-pressure action (operating temperature), the boom cylinder seeps oil, movement speed decreases, and eventually drips oil.

Troubleshooting Process:
– Checked cylinder wiper seal: intact, no damage.
– Checked cylinder piston rod: surface smooth, no scoring.
– Checked cylinder seals: sealing good when cold.
– Measured cylinder chamber pressure at operating temperature: found excessive peak pressure.

Root Cause: Poor heat resistance of the cylinder composite seals causes them to harden and fail at high temperatures.

Required Parts: Boom cylinder seal repair kit.