A contractor reported that a Caterpillar 336 GC excavator's arm (stick) would slowly drift in (towards the tracks) when the safety lever was disengaged. Additionally, the right-hand joystick (stick control) felt "notchy" and did not return to neutral smoothly.
The technician first addressed the joystick issue. He removed the pilot control handle assembly. Inside, the "cam follower" bearing on the stick axis was found to be seized. The operator had been forcing the joystick, which bent the internal "return-to-neutral" shaft. This mechanical bind prevented the spool valve from centering perfectly, causing a slight flow of pilot oil to the main control valve, which explained the drift.
However, after repairing the joystick, the drift persisted, though it was slower. The technician then focused on the hydraulic cylinder. He performed a "Cylinder By-Pass Test." He extended the stick fully, isolated the cylinder lines, and opened the drain plug. Oil continued to weep from the piston side, confirming internal leakage.
The piston seals on the stick cylinder had failed. The bent joystick shaft had masked the symptom of the leaking seal. The repair involved overhauling the pilot joystick with a new bearing kit and shaft. The stick cylinder was removed and sent to a hydraulic shop for resealing (replacing the polyurethane U-cup seals and wear rings). After reassembly, the joystick snapped back to neutral crisply, and the arm held its position.
