Air Brake System
Brake System General Information
A dual air brake system consists of two independent
air systems that use a single set of brake controls.
Each system has its own reservoirs, plumbing, and
brake chambers. The primary system operates the
service brakes on the rear axle; the secondary sys-
tem operates the service brakes on the front axle.
Service brake signals from both systems are sent to
the trailer.
WARNING
Do not operate the vehicle with the front brakes
backed off or disconnected. Backing off or dis-
connecting the front brakes will not improve ve-
hicle handling and may lead to loss of vehicle
control, resulting in property damage or personal
injury.
Brake System Conditions
The warning light and buzzer come on if air pressure
drops below 64 to 76 psi (441 to 524 kPa) in either
system. If this happens, check the dual system air
pressure gauge to determine which system has low
air pressure. Although the vehicle’s speed can be
reduced using the foot brake control pedal, either the
front or rear service brakes will not be operating at
full capacity, causing a longer stopping distance.
Bring the vehicle to a safe stop and have the air sys-
tem repaired before continuing.
IMPORTANT: In the event of a total loss of ser-
vice brakes with full system air pressure, use
the parking brake control valve (yellow knob) to
bring the vehicle to a complete stop in the saf-
est location possible.
Before a vehicle with insufficient system air pressure
can be moved, the spring parking brakes must be
released by applying an external air source at the
gladhands, or by manually caging the parking brake
springs.
WARNING
Do not release the spring parking brakes and
then drive the vehicle. There would be no means
of stopping the vehicle, which could result in se-
rious personal injury or vehicle damage. Before
releasing the spring parking brakes, make the
connection to a towing vehicle or chock the tires.
After correcting the brake system problem, uncage
the spring parking brakes before resuming normal
vehicle operation.
Primary Air Brake System
Loss of air pressure in the primary air system causes
the rear service brakes to become inoperative. The
secondary air system will continue to operate the
front brakes and the trailer brakes (if equipped).
Secondary Air System
Loss of air pressure in the secondary air system
causes the front axle brakes to become inoperative.
The primary air system will continue to operate the
rear service brakes and the trailer brakes (if
equipped).
Brake System Operation
Before driving your vehicle, allow time for the air
compressor to build up a minimum of 100 psi (689
kPa) pressure in both the primary and secondary
systems. Monitor the air pressure system by observ-
ing the dual system air pressure gauge and the low-
air-pressure warning light and buzzer. The warning
light and buzzer shut off when air pressure in both
systems reaches 64 to 76 psi (441 to 524 kPa).
IMPORTANT: Before driving the vehicle, secure all
loose items in the cab so that they will not fly forward
during a full brake application. Make sure all passen-
gers are wearing seat belts.
During normal brake stops, depress the foot brake
control pedal until braking action slows down the ve-
hicle. Increase or decrease the pressure on the
pedal so that the vehicle comes to a smooth, safe
stop. Apply the spring parking brakes if the vehicle is
to be parked.
IMPORTANT: An air brake proportioning system
may be used in tractor air brake systems when
the vehicle is not equipped with antilock braking
system (ABS). When operating in bobtail mode,
the rear brake chambers (on which the axle
load has been greatly reduced) receive reduced
(proportional) air pressure, while the front axle
brake chambers receive full (normal) air pres-
sure. This results in a different brake pedal
"feel," as the pedal seems to require more travel
Brake Systems
12.1
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