A heavy construction crew reported that a Hitachi Zaxis 200-3 excavator's boom would slowly drift down when the safety lever was engaged. Additionally, the joystick control felt jerky, and the machine would "hesitate" when starting to swing.
The technician first tested the pilot system pressure. The gauge showed 30 bar (normal is 55 bar). The pilot pump is gear-driven from the main pump. He removed the pilot pump and found the "Coupling Spider" (rubber dampener) between the pilot pump shaft and the main pump shaft was stripped. This caused the pilot pump to slip, reducing pilot pressure. The low pressure caused the pilot valves to not fully open, explaining the jerky movement.
However, the boom drift was a separate issue. He performed a "Cylinder Drop Test" by extending the boom and closing the isolation valve. The drift stopped, proving the cylinder seals were sound. He then focused on the main control valve. He removed the "Boom Load Check Valve." The check valve seat was scored, and the spring was weak.
The "Load Check Valve" sits just before the boom cylinder port. Its job is to hold the oil column in place when the control valve spool is centered. The scored seat prevented the poppet from sealing, allowing oil to leak back into the valve body.
The repair involved replacing the pilot pump coupler spider and rebuilding the pilot pump. He also replaced the load check valve cartridge. After bleeding the air from the pilot lines and recharging the hydraulic system, the pilot pressure returned to 55 bar, and the boom held its position firmly.
