A sand and gravel operation reported that a Case 721G wheel loader would lift an empty bucket effortlessly, but when attempting to lift a heaped bucket of wet sand, the lift cylinders would stall entirely, and the relief valve would howl. The machine had no steering or transmission issues, and the engine RPM did not lug down, pointing directly to a hydraulic restriction in the implement circuit.
The diagnostic process began by teeing a gauge into the main pump outlet. Under no load, the pump flowed freely. When the loaded bucket was raised, the pressure spiked to the main relief setting of 3,300 psi, but the flow dropped to near zero. This indicated that the pump was building pressure but the fluid was bypassing internally or failing to enter the lift cylinders.
The technician removed the main control valve cover to inspect the lift spool. The spool was coated in a dark, glaze-like varnish and showed visible scoring on its lands. Because the spool could not fully shift to route high-pressure fluid to the lift cylinders, the oil was dumping across the relief valve. But the varnish was a symptom, not the root cause. The scoring was caused by contamination.
The investigation moved upstream to the pilot control manifold. The 721G uses pilot pressure to shift the main spools. A gauge was attached to the lift pilot port. When the joystick was moved, pilot pressure reached 400 psi, but the moment the load hit the cylinders, the pressure bled down to 100 psi. Disassembling the pilot manifold revealed a completely sheared and extruded O-ring on the pilot relief cartridge. The O-ring had been pinched during a previous service, and over time, high-pressure fluid had cut it entirely in half.
When the bucket hit a heavy load, the resulting shock pressure forced the pilot relief valve open. Because the O-ring was destroyed, the valve could not re-seat, allowing the pilot fluid to bypass back to the tank. Without sustained pilot pressure, the main lift spool would slowly drift back to neutral under the mechanical resistance of the load, blocking flow to the cylinders and opening the main relief.
The repair required honing the main lift spool bore to remove the scoring, and installing a new spool to restore the correct clearance. The pilot manifold was rebuilt with a new relief cartridge and O-rings, and the hydraulic system was flushed to remove the varnish particulates. After reassembly, the loader hoisted a full bucket of wet sand smoothly and quickly, with no stall or relief howling.
