AXLE-REAR (With Locking Differential)
To avoid personal injury or property damage,
pay strict attention to the following:
If your vehicle is equipped with a locking
differential, power will be transmitted to the
opposite wheel should one of the wheels slip.
Both wheels must be raised free of the ground
should it be necessary to operate one wheel
with the vehicle stationary; otherwise the
wheel that is not raised will pull the vehicle off
As with any vehicle, care should be taken to
avoid sudden accelerations when both drive
wheels are on a slippery surface. This could
cause both drive wheels to spin, and allow the
vehicle to slide sideways on the crowned
surface of a road or in a turn.
The purpose of this dual system is to provide a means
of stopping the vehicle should a failure occur in one of the two
independent brake systems. In the event air pressure loss
occurs in one system, the remaining system continues to
provide braking action.
The air system splits at the supply reservoir where it
branches into separate service reservoirs. Each service
reservoir supplies air to a separate section of the dual brake
The brake valve is equipped with two separate supply
and delivery systems for service and emergency braking, thus
providing the operator with a graduated control for applying
and releasing the vehicle brakes, even in the event of an air
loss in either the primary or secondary system.
The primary air system on all trucks and tractors
supply air to rear axle brakes and the secondary air system
supplies air to the front axle brakes.
Tractor systems are designed to control the towed unit
from both braking systems. Should failure occur in either
system, control of the trailer service brakes is maintained
through the use of air from the remaining system.
On a straight truck the primary (rear) brakes are
designed to stop the truck in required distance in the event of
air loss in the secondary system.
If a loss of air occurs in the primary system, the air
system on a straight truck is designed to allow the spring
brakes on the rear axle to be applied or released in a
modulated manner at the same time the front service brakes
are applied or released by the operator.
Even though there will be enough braking capability for
emergency stopping, the vehicle should not be operated when
a failure is indicated.
All vehicles are equipped with spring brakes for
parking. The parking system is operated manually by a single
valve, which in the case of a tractor also controls the parking
system on the towed unit. This total vehicle parking
capability, using the parking valve as the master control,
requires the parking brake be released and trailer emergency
valve in the "released" position to permit charging of the trailer
air system. The need for an emergency release reservoir is
eliminated due to its function being provided by either of the
two service brake systems. The parking brakes will
automatically apply only if both service system air pressures
BRAKE ANTILOCK SYSTEM
The antilock system has been added to air braked
vehicles as an aid in helping to keep the vehicle in its traffic
lane during severe braking. It assists the driver by keeping the
wheels from sliding during hard brake applications without
sacrificing stopping distance. Some wheel lock may be
experienced at very low speeds.
The antilock system analyzes wheel speed information
and detects a rapid change in wheel speed such as that
caused during hard braking. It will decrease air pressure
momentarily reducting brake power allowing the wheels on
affected axles to roll more freely, thus preventing the wheel or
brake lock-up which would result in loss of vehicle control as
well as flat spotting of tires. Just as soon as the wheels return
to a controlled percentage of their original speed, the brakes
are automatically reapplied. A sequence of releasing and
reapplying air pressure of the brakes is known as an antilock
cycle and may be repeated several times during a stop,
depending upon the initial speed, road surface and brake
force applied by the driver.
Each cycle can be felt by the driver as a "lurch" or the
same type of feeling that would be obtained when the brake
pedal is rapidly pumped during a stop. On icy roads cycling
may continue long enough that reservoir pressure could be
depleted to a point that the low air pressure buzzer and light
will come on. This is not a cause of immediate alarm since
braking is still available even at this reduced pressure.
Poor brake adjustment will aggravate air consumption
since long chamber strokes will require more air to be used
during each cycle. This means that reservoir air pressure will
be depleted faster than would be the case with good brake
adjustment and short chamber strokes.