A wheel loader was brought in because it kept drifting to the right, but only when the bucket was loaded. If the operator was driving around the yard empty, the machine tracked straight. But the moment they scooped a full bucket of gravel and drove away, the steering wheel would slowly turn to the right, forcing the operator to constantly fight the wheel.
The shop had already replaced the steering orbital unit (the valve under the steering wheel) and the priority valve, assuming the hydraulics were bypassing internally. We put a gauge on the system, and the standby pressure was perfectly normal. Because the problem only happened under a heavy front-end load, we had to look at the steering cylinder's mechanical geometry.
When a loader picks up a heavy load, the front frame pivots downward slightly on the center articulation joint, placing a massive tension force on the frame. This tension tries to pull the left and right front frame halves apart, which in turn puts an extreme pulling force on the tie-rods and the steering cylinder. We disconnected the steering cylinder rod and found the issue. The piston inside the cylinder had a microscopic crack in the polyurethane seal. Under normal pressure with no load, the seal held fine. But when the bucket was loaded, the physical tension on the frame pulled the cylinder rod outward with thousands of pounds of force. This force compressed the cracked seal enough to allow high-pressure oil to sneak past the piston into the return side, slowly collapsing the cylinder and turning the wheels. We replaced the steering cylinder, and the machine tracked straight with 20 tons in the bucket.
