How to troubleshoot the faults of the brake caliper of a loader

A ZL40 loader once experienced difficulties in starting, weak driving power, veering off course after braking, and a small amount of oil fumes coming out of the left front wheel with a burnt smell after the engine was turned off during operation. Upon inspection, it was found that the brake disc and the right friction plate were not completely separated, and the piston of the right brake caliper did not return to its original position.

The ZL40 loader adopts a double-pipeline pneumatic oil disc braking system. Four brake calipers are installed on the disc brake 7, and they are placed in pairs on the ground. When the brake pedal is depressed, the high-pressure gas from the air compressor l pushes the gas piston of the gas-liquid master pump 4, and the gas piston then pushes the oil piston. Let the brake fluid enter the brake caliper piston chamber of the disc brake through the oil pipe and push the piston out. The piston drives the friction plates to clamp the brake discs, causing the wheels to brake.

There are four possible causes for the brake caliper of the disc brake not to return to its original position: First, the exhaust port of the foot brake valve 5 is blocked, and high-pressure gas can be discharged into the atmosphere, causing the spring of the pneumon-hydraulic master cylinder to fail to return to its original position, resulting in the brake fluid pushing the piston of the brake caliper of the disc brake to the dead and preventing it from returning to its original position. The second reason is that the oil pipe from the pneumon-hydraulic master cylinder to the disc brake is clogged, causing the piston of the brake slave cylinder to fail to return to its original position. Thirdly, the spring of the pneumatic-hydraulic master cylinder itself cannot return to its original position. Under the action of high-pressure gas, the brake fluid cannot return to the oil storage chamber of the pneumatic-hydraulic master cylinder, thus causing the piston of the brake slave cylinder to fail to return to its original position. The fourth issue is that the brake caliper piston of the disc brake gets stuck due to the brake fluid being too dirty or the piston rusting and thus cannot return to its original position.

When diagnosing the fault, I first stepped on the foot brake valve. After releasing the pedal, I heard an exhaust sound, thus ruling out the first possibility. Then, I unscrewed the oil pipe joint from the pneumon-hydraulic master cylinder to the brake caliper of the disc brake. I stepped on the foot brake valve several times and found that brake fluid was spraying out from the joint and there was a relatively high pressure. This ruled out the second and third possibilities. Finally, we determined that the fault lay in the brake caliper.

The method to eliminate the fault is to remove the disc brake clamp 1, take off the friction plate 4 and the dust ring 3, and reconnect the oil pipe joint. We hoped to push the piston out under the high pressure of the brake fluid, but several times when we stepped on the brake valve under high pressure, it failed. The piston 5 of the sub-pump could neither return to its original position nor be pushed out, and was completely stuck. So, we removed the hydraulic cylinder head 6 of the clamp and pushed out the sub-pump piston. Then, it was cleaned with diesel, sanded with 0# sandpaper and further cleaned, and a new rectangular sealing ring 2 was replaced. After reassembling and testing the machine again, the fault has been eliminated.