A construction site reported that a Hyundai R210LC-9S excavator's boom would slowly sink (drift) to the ground when the engine was turned off. The machine also had a slight "jerk" when lowering the boom.
The technician diagnosed the hydraulic circuit. The boom circuit on the R210LC-9S uses a "Load Holding Valve" (a pilot-operated check valve) mounted on the main control valve block. This valve locks the oil in the cylinder when the control lever is in neutral.
He removed the "Boom Load Holding Valve Cartridge." Upon inspection, he found a small "Crack" on the valve seat. The crack prevented the valve from sealing 100%, allowing oil to bypass the seat and leak back to the tank. This caused the drift.
The "Jerk" during lowering was caused by the "Pilot Piston" inside the valve. The piston was sticking due to varnish buildup. When the operator moved the joystick to lower the boom, the pilot pressure couldn't push the piston open smoothly. It would stick, then suddenly pop open, causing the jerky movement.
Furthermore, the technician found the "Boom Cylinder Piston Seals" were worn. He pressurized the cylinder and found oil weeping from the rod gland. The leakage contributed to the thermal instability of the oil.
The repair involved replacing the load holding valve cartridge and the boom cylinder seals. The hydraulic tank was drained and refilled. After bleeding the air, the boom held its position solidly, and the lowering operation became smooth.
