A landscaping contractor reported that a Sany SY75C Pro mini excavator's boom would drop rapidly when the safety lever was engaged, and the pilot controls felt spongy. The symptoms appeared after the machine had been sitting idle for a week.
The technician first bled the air from the hydraulic system. He found that the pilot line had a loose fitting, which was allowing air to be sucked into the pump. After tightening the fitting and bleeding the air, the sponginess improved, but the boom drop persisted.
To diagnose the drift, he performed a "Reverse Flow Test." He raised the boom, turned off the engine, and loosened the return line from the boom cylinder. Oil continued to flow out even when the control valve was in neutral. This indicated that the oil was bypassing the "Load-Holding Check Valve" inside the main control valve.
He removed the main control valve and disassembled the boom section. He found a small piece of rubber seal lodged in the seat of the load-holding check valve poppet. This prevented the valve from sealing, allowing the weight of the boom to push oil back to the tank. The piece of rubber matched the seal from the pilot line fitting that had come loose earlier.
The repair involved removing the debris, cleaning the check valve seat, and installing a new seal kit for the pilot line fitting. The main relief valve was also reset to the correct pressure. After reassembly, the boom held its position, and the pilot controls felt firm and responsive.
