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).
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,
causing a longer stopping distance. Bring the vehicle
to a safe stop, and have the air system repaired be-
fore 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.
On tractor-trailer vehicles, if both the primary and
secondary systems become inoperative, the trailer
service brakes or spring parking brakes will automati-
cally apply when air pressure drops below 35 to 45
psi (242 to 310 kPa). The tractor spring parking
brakes will automatically apply when air pressure
drops below 20 to 30 psi (138 to 207 kPa). On
straight trucks, spring parking brakes will apply when
air pressure drops below 20 to 30 psi (138 to 207
kPa). Do not wait for the brakes to apply automati-
cally. When the warning light and buzzer first come
on, immediately bring the vehicle to a safe stop. Be-
fore continuing operation of the vehicle, correct the
cause of the air loss.
Before the vehicle 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 (cage) the spring parking brakes
and then drive the vehicle. There would be no
means of stopping the vehicle, and this could
result in serious personal injury or vehicle dam-
age. Before releasing (caging) 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.
Brake System Operation
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 passengers 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
is used in tractor air brake systems when the
vehicle is not equipped with ABS. When operat-
ing in bobtail mode, the rear brake chambers
(whose axle load has been greatly reduced) re-
ceive reduced or proportional air pressure, while
the front axle brake chambers receive full (nor-
mal) air pressure. This results in a different
brake pedal "feel," as the pedal seems to re-
quire more travel and/or effort to slow or stop
the vehicle. However, the air brake proportioning
system actually improves vehicle control when
the tractor is in the bobtail mode of operation.
When the tractor is towing a trailer, the rear
brake chambers will receive full (normal) appli-
cation air pressure.
NOTE: If equipped with main and auxiliary
transmissions, do not shift both transmissions
into neutral while the vehicle is rolling. Shifting
both transmissions back into gear would be diffi-
cult while the vehicle is rolling.
The trailer brake hand control valve (
Fig. 6.2
) actu-
ates the trailer service brakes independently from the
vehicle chassis service brakes. The valve can be
partially or fully applied, but in any partially-on posi-
tion it will be overridden by a full application of the
foot brake control pedal. Moving the valve handle
clockwise actuates the trailer brakes, while moving it
counterclockwise releases the trailer brakes. The
valve handle is spring-loaded and returns to it’s origi-
nal, no-air application position when released.
Steering and Brake Systems
6.2