Fault Phenomenon: Dynapac F1800C paver after extending the screed to its width, it automatically retracts during operation.

Troubleshooting Process:
– Checked hydraulic lock: bi-directional hydraulic lock spool internal leakage.
– Checked cylinder: extension cylinder piston seal failed.
– Checked control valve: spool not fully centered.
– Checked load: excessive material resistance.

Root Cause: The bi-directional hydraulic lock spool for the screed extension cylinder is worn and leaking internally, unable to lock cylinder pressure, causing retraction under material resistance.

Required Parts: Replace bi-directional hydraulic lock assembly.