Spring-type brakes in this system serve two purposes: first, as a parking brake and second, as an emergency system.
If a failure occurs in the primary circuit and a brake application is made, control air from the secondary side of the foot valve is directed to a spring-brake modulator (Fig. 36).
As there is no primary supply air to maintain balance in the modulator valve (due to the primary circuit failure), the modulator valve then exhausts air pressure from the spring-brake circuit. The amount of air released is equal to the amount of air applied by the foot valve. Release of air in the spring-brake circuit causes the drive axle to brake using spring-brake pressure. When the brake is released, supply air from the secondary circuit returns the spring brakes to an off position.
Brake applications can be repeated until all the air from the secondary circuit is lost, but as air pressure drops below 70 psi (483 kPa), the spring brakes won’t return to full off position - in fact, they will start to drag. At about 20 psi (138 kPa), the spring-brake control valve on the dash exhausts the remaining air in the spring-brake circuit, and the spring brakes are fully applied. The only way the vehicle can be moved after all air is lost is to repair the damaged circuit and recharge the system, or use the wind-off bolts to compress the power spring. This process is called caging the brakes