1. Handbrake Valve Leakage
When the handbrake valve is set to the "parking brake" position, there is continuous air leakage from the exhaust port of the handbrake valve. Generally, this occurs due to an imperfect seal between the valve's intake valve and its seat, or due to foreign objects between the valve and the seat, or damage to the intake valve sealing component. Replacing the intake valve seal with a repair kit will resolve the issue.
During parking braking, handbrake valve leakage does not cause other faults. However, when the handbrake valve handle is set to the "travel" position, if there is air leakage from the handbrake valve, it may result in the fault of the rear axle wheels being "gripped tightly" while driving. This is generally caused by poor contact and sealing between the valve stem and the air valve of the handbrake valve. The reason for the poor sealing at the exhaust port could be damage to the sealing components or the presence of foreign matter or oil contamination between the valve rod 7 and the air valve 10, causing the exhaust port to fail to seal properly. Disassembling, inspecting, cleaning, or replacing the intake valve seal can eliminate the fault.
2. Excessive Charging Time for the Handbrake Air Tank
Before starting the car, the main brake air tank should first be fully charged with pressure. The warning light turning off and the buzzer stopping indicate that the main brake (front brake air tank and middle/rear brake air tank) has been sufficiently pressurized. If, at this point, the handbrake low-pressure warning light illuminates when the handbrake valve is placed in the "travel" position, it indicates insufficient air pressure in the handbrake air tank, meaning the spring brake chamber has not fully released the brakes. Forcing the car to start under these conditions would cause the middle/rear axle to produce a "gripped" braking phenomenon. Therefore, the car should only start after the warning light has turned off.
Under normal circumstances, after the main brake air tank is fully charged (the main brake low-pressure warning light turns off), the handbrake air tank quickly fills up (the handbrake low-pressure warning light turns off). If this charging time is excessively long (the handbrake low-pressure warning light does not turn off promptly), it suggests there is an issue with the check valve (or overflow valve) bridging the middle/rear brake circuit and the handbrake circuit. This valve is a one-way overflow valve that should open and rapidly charge the handbrake circuit when the middle/rear brake circuit reaches 6.5 bar. If this pressure setting is adjusted too high, causing the valve to get stuck (or frozen) and fails to open, it will lead to the fault of excessive charging time for the handbrake circuit. Disassembling and adjusting this valve will resolve the problem.
3. Spring Energy Storage Chamber Air Leakage
Damage or scoring of the piston seal ring in the spring energy storage chamber, or scoring of the cylinder in the chamber, can all cause air leakage in the chamber. Since the spring energy storage brake chambers of the middle/rear axle wheels are all interconnected via air lines, any single chamber leakage will reduce the air pressure in all chambers, leading to the "gripped" result during driving. In the event of such a fault, the leaking brake chamber needs to be disassembled and repaired. When disassembling and installing the spring energy storage brake chamber, it must be done on a press to ensure safety.
4. Handbrake Relay Valve Leakage
When the vehicle is in the parking brake condition, relay valve leakage will not cause faults in the vehicle. This is clearly caused by damage to the sealing component of the brake valve's intake valve or the presence of foreign objects or impurities between the valve and the valve seat, resulting in poor valve sealing.
When the vehicle is in motion, continuous air leakage from the exhaust port of the relay valve is obviously due to poor sealing between the exhaust valve and the piston. This will cause insufficient air pressure in the spring energy storage brake chamber, leading to the fault of "gripped" service braking.