A sand and gravel pit reported that a Hyundai HL940 wheel loader had weak lifting power and the boom would slowly drift down when the engine was shut off. The operator had to "feather" the joystick constantly to hold the bucket height while traveling.
The technician first tested the main hydraulic relief pressure, which was at spec. The drift was isolated by raising the boom and shutting off the machine. The boom dropped, but the bucket cylinders held. This suggested an external leak or a valve issue in the boom circuit. A visual check showed no external leaks on the boom cylinders.
The technician then removed the main control valve cover. Upon inspection, a hairline crack was found on the internal wall of the valve housing, between the boom spool bore and the return port. When the boom was raised, high-pressure oil was escaping through the crack directly to the tank, causing the weak lift. When the spool was centered, the oil trapped in the cylinder would bleed past the crack, causing the drift.
Additionally, when the boom cylinders were disassembled to check for contamination, the piston seals were found to be flattened. While this was not the primary cause of the drift (the valve was), it contributed to the weak lift. The repair involved replacing the main control valve assembly and rebuilding the boom cylinder seals. The root cause of the cracked valve was traced to a previous impact on the loader's chassis, likely from hitting a large rock, which stressed the casting.
