AIRBRAKE SYSTEM - CONTINUED
BRAKE PEDAL VALVE
The brake pedal valve is a dual-circuit control valve mounted to the inside of the firewall. It meters air pressure from the
reservoirs through the system to apply the brakes when the driver depresses the pedal and releases pressure when the
driver releases the pedal.
The two relay valves are piston-type air operated valves mounted to the vehicle's left side frame rail. They are designed to
speed the application and release of air pressure to the rear brakes and to meter the same pressure to each of the two
brakes they service. Air pressure is also metered through the relay valves according to the modulator valves in the brake
The quick-release valve is a diaphragm-type air operated valve mounted to the vehicle's front crossmember. It is designed
to speed the application and release of air pressure to the front brakes and is metered by the front modulator valve of the
RELAY QUICK-RELEASE VALVES
The two relay quick-release valves are piston-type air operated valves mounted to the vehicle's left side frame rail. They
are designed to speed the application and release of air pressure from the spring brake chambers.
The inversion valve is a piston-type air operated valve mounted on the center frame crossmember. It is designed to supply
and exhaust air pressure to the spring brake chambers in proportion to driver application of the front (secondary) brakes
when there is a pressure loss in the rear (primary) brake portion of the brake system.
AIRBRAKE CHAMBERS (FRONT AND REAR)
The airbrake chambers transfer energy from compressed air into mechanical force and motion using a diaphragm and
pushrod. One chamber is mounted at each brake assembly. Air pressure entering the chamber when the brakes are
applied forces the pushrod out, applying the brakes. When the brakes are released, air pressure leaves the chamber
allowing the brake return springs and return spring in the chamber to release the brakes. A spring brake chamber is
mounted on each forward rear axle brake chamber to apply the brakes if air pressure is lost, using a powerful spring
pushing the brake chamber pushrod out. Air pressure entering the spring brake chamber forces the spring back, releasing
The brake assembly is a cam-actuated conventional brake assembly mounted to each axle. Using the pressure of shoes
against the brakedrums, they slow the vehicle when the brakes are applied, and return springs release the shoes when the
brakes are released. A slack adjuster mounted on each camshaft is acted upon by the brake chambers to allow even
braking by the brake assemblies.