A heavy civil contractor reported that a Kobelco SK350LC-8 excavator had a specific issue: the boom would lower slowly, and when the lever was released, the boom would drift down approximately 20 cm before stopping. The boom lift function (raising) was strong and normal.
The technician identified this as a "regeneration circuit" issue. When an excavator boom is lowered, the weight of the boom assists the downward movement, potentially creating a vacuum (cavitation) on the rod side of the cylinder. To prevent this, the hydraulic system uses a regeneration valve to route oil from the piston side back to the rod side. If this valve fails, the boom lowers slowly due to a lack of return oil flow.
The technician first tested the "pilot check valve" on the boom cylinder. It held pressure, ruling out a cylinder seal leak. The focus shifted to the main control valve. The boom control valve block was removed. Inside, the "boom regeneration check valve" was found to be stuck in the closed position by a tiny piece of metal debris. This blocked the free flow of oil during lowering, causing the slow speed. The drift was caused by a score mark on the main boom spool. The score allowed pressurized oil to leak past the spool into the tank return line, but only when the spool was in the neutral position.
The repair involved cleaning the regeneration valve seat with a polishing stone and replacing the boom control spool. The hydraulic tank was drained and cleaned to remove the source of the metal debris. After reassembly, the boom lowered smoothly at full speed, and the drift was eliminated.
