A heavy civil contractor reported that a Kobelco SK350LC-10 excavator was swinging very slowly, and the swing brake would not hold the house in position on a slope. The machine would slowly creep (rotate) when parked on an incline, even with the safety lock lever engaged.
The excavator utilizes a pilot hydraulic system to control the main control valves and the swing brake. The technician first checked the pilot pressure at the pump. It was reading a steady 500 psi, which is standard. However, when the swing lever was actuated, the pilot pressure dropped to 200 psi. This indicated a flow restriction between the pilot pump and the swing control circuit.
The pilot filter, located near the hydraulic tank, was removed for inspection. It was completely clogged with a grey, pasty substance-later identified as aluminum shavings from a failing hydraulic pump swash plate bearing that had happened months prior. The clogged filter was starving the pilot circuit of flow when demand was high.
Furthermore, the swing brake is a spring-applied, hydraulically released multi-disc brake. To release the brake, pilot pressure pushes against a spring. The pilot line for the swing brake passes through a solenoid valve. The technician removed the swing brake solenoid valve and found the spool was stuck in the "open" position due to the same metallic paste. This meant the brake was constantly receiving pilot pressure (even when the lever was in neutral), keeping the brake partially released. This caused the slow swing (dragging brake) and the drift (brake not fully setting).
The repair involved replacing the pilot filter element and flushing the pilot lines with clean oil. The stuck swing brake solenoid was disassembled, cleaned, and the spool was polished. The main hydraulic pump was also inspected for further wear, as the source of the contamination needed to be addressed. After replacing the pilot filter and cleaning the solenoid, the swing speed returned to normal, and the brake held the machine securely on steep slopes.
