Fault Phenomenon: Schwing S700 hydraulic system functions normally when cold. During prolonged pressure holding (operating temperature), the hydraulic system leaks internally, the boom slowly sinks, and eventually becomes uncontrollable.

Troubleshooting Process:
– Checked hydraulic lock: good sealing, no internal leakage.
– Checked counterbalance valve: cone valve seals well, no wear.
– Checked cylinder seals: piston seals good, no scoring.
– Measured system internal leakage at operating temperature: found excessive internal leakage.

Root Cause: The counterbalance valve spool expands due to heat, causing the sealing clearance to increase.

Required Parts: Counterbalance valve assembly.