A landscaping contractor reported that a Sany SY75C mini excavator would slowly "weep" down (drift) when the boom was raised and held in place. Additionally, the boom lift speed was noticeably slower than the stick function, and the joystick felt "spongy" or soft when moved to the boom raise position.
The technician first tested the main hydraulic pump flow. The pump was healthy. He then tested the load holding check valves in the boom circuit, which were functional. The "spongy" joystick feel pointed to a pilot system issue. He tested the pilot pressure, which was fluctuating between 30 and 40 bar (should be steady at 35-40 bar). He bled the pilot system, but the fluctuation returned.
Investigating further, he traced the pilot line to the main control valve. A visual inspection revealed a hairline crack on the exterior of the main control valve housing, specifically where the pilot pressure line connects. This crack was allowing air to be sucked into the pilot system when the engine was running, causing the spongy feel. But more critically, the crack was located right next to the "Boom Port A" passage.
When the boom was raised, high-pressure oil from the cylinder was leaking through the crack into the pilot return line, causing the drift. The crack had likely started as a casting defect or stress fracture from previous impact. The repair involved removing the main control valve and sending it to a specialized machine shop for "Lock-N-Stitch" threading repair. The pilot port was re-drilled and tapped with a helicoil insert. After reinstallation and bleeding the pilot system, the drift stopped, and the joystick response became firm and precise.
