An excavator that slowly drifts down or drifts out of position when parked is usually suffering from leaky cylinder piston seals. But if you replace the seals and the drift continues, you need to look at the pilot circuit-specifically, the pilot drain line.
Pilot pressure is constantly supplied to the joysticks. When a joystick is centered, the pilot oil must drain back to the tank through a small, filtered drain line. If this drain line is restricted-often by a crushed hose or a filter screen plugged with Teflon seal debris-the pilot oil cannot escape.
This creates a slow, creeping backpressure in the pilot circuit. Even though the joystick is centered, this backpressure pushes against the small end of the main control valve spools. Over time, it overcomes the light centering springs and slowly strokes the main spool, sending pilot oil to the boom or stick cylinder. The machine will slowly drift as long as the engine is running. The tell-tale diagnostic: if the machine holds perfectly still when the engine is shut off, but drifts only when the engine is idling, you have a blocked pilot drain line building up ghost pressure, not a leaky cylinder.
