Fault Phenomenon: Zoomlion ZLJ5600THB brakes function normally when cold. While driving downhill (operating temperature), brakes fail, the pedal goes to the floor, eventually leading to an accident.

Troubleshooting Process:
– Checked brake pads: pad thickness normal, no oil contamination.
– Checked brake drums: drums no cracks, no overheating discoloration.
– Checked brake air lines: no air leaks, no aging.
– Measured brake wheel cylinder pressure at operating temperature: found pressure cannot be built up.

Root Cause: The brake wheel cylinder piston cup expands due to heat, causing a seal failure and air leak.

Required Parts: Brake wheel cylinder repair kit.