A wheel loader had a maddening problem. The bucket would tilt back and hold a load perfectly fine when the boom was resting on the ground. But as soon as the operator raised the boom past 45 degrees, the bucket would slowly drift forward, dumping the load. The shop replaced the bucket cylinder piston seals, but the drift persisted.
We looked at the pilot hoses that run from the joystick valve along the boom to the main control valve. Because the boom raises and lowers constantly, these pilot hoses are subjected to severe bending at the boom pivot pin. Over time, the inner rubber liner of the hose delaminated from the outer steel braid.
When the boom was down, the hose was relatively straight, and the delaminated rubber laid flat, allowing pilot oil to flow normally. When the boom raised, the hose bent sharply at the pivot. The inner flap of delaminated rubber acted like a check valve, folding over and completely blocking the pilot oil return passage. With the pilot fluid trapped, the main control spool was held slightly open, slowly allowing the bucket to drift. Replacing the $40 pilot hose at the pivot fixed the$2,000 cylinder ghost.
