A landscaping contractor reported that a Sany SY75C Pro mini excavator's boom would "chatter" (shake) when lowering slowly, and the boom would drift down when the engine was off.
The technician first bled the air from the hydraulic system. He then focused on the "Boom Control Block." On the SY75C, the boom circuit uses a "Load Holding Valve" (a pilot-operated check valve) to lock the cylinder in place. He removed the cartridge. The "Pilot Piston" inside the valve was stuck due to varnish buildup.
The load holding valve works by using pilot pressure to open the check valve. If the piston sticks, the valve opens and closes rapidly (chattering) as the pressure fluctuates. This caused the jerky movement. The "Load Drop" occurred because the stuck piston prevented the valve from fully seating (closing) when the joystick was released, allowing oil to bypass.
Furthermore, he found the "Pilot Filter" was loose. This allowed air to enter the pilot system. The air bubbles compressed and expanded rapidly, exacerbating the chatter.
The repair involved soaking the load holding valve in solvent to dissolve the varnish. The pilot filter was tightened. The hydraulic system was purged of air. After reassembly, the boom lowered smoothly and held its position solidly.
