A construction site reported that a Lonking LG855D wheel loader had a "soft" brake pedal. The operator had to pump the pedal twice to get the machine to stop. There was also a hissing sound from the brake area when the pedal was pressed.
The technician diagnosed the "Air-Over-Hydraulic" brake system. The system uses an air booster to push the master cylinder piston. He checked the air pressure (0.7 MPa), which was normal. He then checked the brake fluid level, which was low. He found a leak at the "Brake Master Cylinder" push rod seal. The leak allowed air into the hydraulic system, causing the soft pedal.
However, the hissing sound was a separate issue. He listened to the "Air Booster" (brake valve). The booster was leaking internally. Upon removing the booster, he found the "Housing Wall" was cracked. The crack was caused by a previous impact (likely a rock hitting the underside of the chassis). The crack allowed compressed air to escape, reducing the force applied to the master cylinder.
The repair involved replacing the master cylinder seal kit and the air booster assembly. He also noticed the "Brake Fluid Reservoir" breather cap was clogged. This created a vacuum as the fluid level dropped, pulling air past the seals. The cap was cleaned. The system was bled by opening the bleeder screws on the four calipers and pumping the pedal until clear fluid flowed. After bleeding, the brake pedal became firm, and the hissing stopped.
