A logistics warehouse reported that a Heli CPCD30 diesel forklift had intermittent hard steering. When turning the steering wheel fully to the left or right, the steering would suddenly become stiff. The operator had to "pump" the steering wheel to get it to turn.
The technician first checked the steering fluid level, which was correct. He then checked the "Steering Priority Valve." In this forklift, the steering pump shares flow with the lift cylinders. A priority valve ensures the steering gets oil first. He removed the priority valve cartridge. The spool was covered in varnish and was sticking. The varnish was caused by overheated oil.
He cleaned the valve spool with a solvent. However, the intermittent issue persisted. He then focused on the "Steering Cylinder." He jacked up the rear of the forklift and turned the wheels manually. He heard a "gurgling" sound. This indicated air in the cylinder. He found a small leak at the "Cylinder Rod Seal." The leak was allowing air to be sucked into the system when the wheels were turned fully (creating a vacuum in the cylinder).
The "hard steering" occurred because the air compresses inside the cylinder, preventing the oil from pushing the piston. The "sticking" priority valve compounded the issue by not supplying enough flow to overcome the air compression.
The repair involved honing the steering cylinder bore, replacing the rod seal and o-rings, and cleaning the priority valve. He bled the steering system by turning the wheels fully left and right with the engine at idle for 10 cycles. The steering became smooth and responsive.
