Installation of parking brakes and piping arrangements into a vehicle air brake system will vary, depending on the vehicle make.
Control valves will vary, depending on the manufacturer and type of piping arrangements.
The type of spring-loaded valve shown (Fig. 32) requires that the driver push the button to release the parking brakes. If the air pressure in the system falls below approximately 70 psi (483 kPa), the spring brakes may begin to drag and if it falls between 20-45 psi (138-310 kPa), will fully apply. On many vehicles the parking brake control valve on the dash will close, however some valves may never close. The important thing is that the spring brakes are fully applied before the air is depleted. Always ensure the spring brakes have been fully applied. Similar types of spring-loaded valves require the driver to pull the button out to release the parking brakes.
Note: On some newer models the park brake button will not pop out automatically. However, the brakes will still apply.
Note: There is a toggle control valve in use that does not have an automatic brake application feature. The park brakes will gradually apply as the air pressure is depleted, however, the control valve will not move. When air pressure is restored, the park brakes will release if the toggle valve is not manually moved to the park brake “on” position.
CAUTION: Compounding the brakes happens when a service brake application is made with the park brake still applied. This can result in damaged brake components and possibly brake failure. To avoid compounding, the park brake should be released before a foot brake application is made.
Note: An anti-compound line (see Fig. 34) is sometimes installed between the delivery side of the primary circuit relay valve and the control side of the relay valve operating the spring brakes. When a brake application is made, the relay valve operating the spring brakes gets a signal from the service brake to release the spring brakes with the same amount of pressure applied to the service brakes. This prevents service-brake and spring-brake pressure from compounding on the brake linkages.
System charged - normal running condition
With air pressure of 70 psi (483 kPa) or greater acting upon the emergency diaphragm (A) and piston (B) in the spring hold-off cavity, the spring (C) is fully compressed and the piston (B) is held in the released position. This does not affect the service diaphragm (D) or service push plate and rod (E).
Park and emergency application
When the driver operates the park control valve, air is exhausted from the spring hold-off cavity. The spring (C) is now allowed to extend, forcing the piston (B) and the diaphragm (A) forward. The piston (B) forces the service diaphragm (D) and service push plate and rod (E) forward compressing the return spring (F) and applying the brakes. To release the park application, the park control valve is placed in the “release” position, releasing the brakes as described under “System charged - normal running condition.”
During a controlled service brake application, air pressure enters the service port and acts upon the service diaphragm (D), which forces the service push plate and rod (E) forward, applying force to the slack adjuster. The slack adjuster rotates the camshaft and applies the brakes. The emergency spring is held in the compressed position by air pressure in the spring hold-off cavity.