COMPRESSED AIR AND BRAKE SYSTEM OPERATION (Contd)
PEDAL INTO VALVE - Allows air pressure from primary constant pressure system to flow into primary signal
system when operator depresses brake pedal.
DOUBLECHECK VALVE #1 - Serves two functions:
Allows system to receive signal pressure from either pedal valve or, when towed, from brake system of
Serves as a tee between front and rear primary signal lines.
FRONT SERVICE COUPLING - When vehicle is being towed, coupling is connected to towing vehicle so that the
brake systems of the two vehicles work together.
ONE-WAY CHECK VALVE - Allows air pressure to flow into primary reservoir but prevents it from coming out if
constant pressure system fails or engine stalls.
PRIMARY AIR RESERVOIR - Stores enough air pressure so the operator can make five normal brake
applications before running out of air if constant pressure fails or engine stalls.
DOUBLECHECK VALVE #2 - Allows either primary or secondary signal air pressure to activate stoplight switch
while keeping the two systems separate.
STOPLIGHT SWITCH - As the brake pedal is depressed, switch receives an air pressure signal which closes
electric contacts turning on stoplight.
REAR SERVICE COUPLING When towing another vehicle, coupling is connected to towed vehicle so that the
brake system of the two vehicles work together.
INTERMEDIATE FRONT BRAKE CHAMBERS Converts air pressure to mechanical force which applies
intermediate rear service brake.
PRIMARY RESERVOIR DRAINVALVE - Provides a drain for moisture and air from primary air reservoir.
REAR RELAY VALVE - Serves three functions:
Boosts signal air pressure to rear brake chambers. Air signal from brake pedal opens valve to route constant
air pressure to rear brake chambers.
Regulates signal air pressure from brake pedal to rear brake chambers so operator has control over amount
of braking. Regulates amount of constant air pressure going to brake chambers as the operator depresses the
Releases air pressure in rear brake chamber directly to vent system when brake pedal is released.
REAR FRONT BRAKE CHAMBERS - Converts air pressure to mechanical force which applies rear service