A lumber yard reported that a Linde H40D diesel forklift made a loud "clunking" noise every time the operator tilted the mast forward or backward. Additionally, the mast would slowly drift backward when carrying a heavy load on an incline. The steering also felt spongy.
The technician performed a visual inspection of the mast tilt cylinder. The pivot pins connecting the cylinders to the mast frame were visibly loose. The bronze bushings inside the cylinder eye mount had worn through, creating metal-to-metal contact. This was the source of the "clunking" noise. When the tilt function was activated, the cylinder would lunge forward within the oversized mounting hole before actually moving the mast.
However, the drift issue was hydraulic. The tilt cylinder rods were dry, so the seals were likely intact. The technician tilted the mast fully back and cracked the hydraulic line at the cylinder. A significant amount of air bubbles sputtered out. This air was causing the spongy steering and the drift, as air is compressible. The source of the air was traced to a loose fitting on the hydraulic tank return line, which was drawing in air as the fluid returned to the tank.
The repair involved pressing out the old bushings from the tilt cylinder mounts and pressing in new oversized bushings to take up the wear. The loose return line fitting was tightened. The hydraulic system was then "bled" by cycling the tilt and steering functions fully several times with the engine at idle to purge the air. The clunking noise disappeared immediately, and the drift stopped once the air was removed from the closed hydraulic circuit.
