An aggregate processing plant reported that a Cat 966M wheel loader was taking over ten seconds to raise a loaded bucket, and once raised, the boom would slowly drift down by six inches when the engine was idling. The machine had previously suffered a hydraulic oil cooler leak that resulted in the system overheating to 120°C, but the cooler had since been replaced.
The technician first performed a cylinder drift test. The boom was raised, the engine shut off, and the lever moved to float. The boom did not drop, isolating the issue to the main control valve rather than the cylinders holding a load. However, when the engine was restarted and the system pressurized, the boom immediately began to drift down.
A flow meter was plumbed into the main pump outlet. When the lift lever was activated, the pump reached the relief pressure of 3,500 psi, but the flow dropped to nearly zero, indicating a massive internal restriction or bypass. The main lift control valve was removed and disassembled. The main lift spool was heavily jammed with chunks of melted, vulcanized rubber.
The rubber debris was traced back to the boom cylinders. During the previous overheating event, the extreme 120°C oil temperature had caused the main piston seals in the boom cylinders to melt and extrude. The degraded rubber had circulated through the system and packed into the fine tolerances of the main lift spool, preventing it from shifting fully into the hold position. This allowed pressurized fluid to bypass the spool lands and return to the tank, causing the drift and the inability to build effective flow for lifting.
The repair required removing and completely rebuilding both boom cylinders with high-temperature Viton piston seals and new wear bands. The main control valve body was chemically flushed in an ultrasonic bath to remove all traces of the melted rubber. The hydraulic tank was drained and steam-cleaned, and the return filter housing was inspected for warpage. After refilling with fresh ISO 68 hydraulic oil and bleeding the cylinders, the loader lifted a full bucket in less than three seconds and held its position rock-solid with zero drift.
