COMPRESSED AIR AND BRAKE SYSTEM OPERATION (Contd)
The secondary constant pressure system is made up of the following components:
LOW AIR PRESSURE SWITCH - Activates warning buzzer and warning lights when air pressure goes below 60
psi (414 kPa).
PEDAL VALVE - Allows air pressure from secondary constant pressure system to flow into secondary signal
system when operator depresses brake pedal.
SECONDARY AIR PRESSURE GAUGE - Indicates amount of air pressure in secondary system.
ONE-WAY CHECK VALVE - Allows air pressure to flow into secondary reservoir but prevents it from coming out if
constant pressure system fails or engine stalls.
SECONDARY 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.
INTERMEDIATE REAR BRAKE CHAMBERS - Converts air pressure to mechanical force which applies
intermediate rear service brake.
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.
SECONDARY AIR RESERVOIR DRAINVALVE - Provides a drain for moisture and air from secondary air
FRONT RELAY VALVE - Boosts signal air to rear brake chambers; regulates air pressure to rear brake chambers
so operator has control over amount of braking; and releases air pressure to rear brake chambers directly to vent
when brake pedal is released.
STOPLIGHT SWITCH - As the brake pedal is depressed, switch receives an air pressure signal at electrical
contacts which close to activate circuits to taillights.
DOUBLECHECK VALVE #2 - Allows either primary or secondary signal air pressure to activate stoplight switch
while keeping the two systems separate.
REAR-REAR BRAKE CHAMBERS - Converts air pressure to mechanical force which applies rear-rear brakes.