Air Brake Manual

Brake Adjustment
On vehicles equipped with hydraulic brakes it is
possible to pump the brake pedal to compensate for
brakes that are out of adjustment. This is not
possible with a vehicle equipped with an air brake
system with manual slack adjusters. When the
brakes are adjusted by slack adjusters, the shoes
are moved outwards. This brings them as close to
the drums as possible minimizing the amount of free
travel when the brakes are applied. It also reduces
the volume of air used to apply the brakes.
Brake adjustment (push rod travel) must be checked
as part of the pre-trip air brake inspection (Section
9).
S-cam Brake
The following are recommended steps to determine
if an S-cam brake with manual or automatic slack
adjusters requires adjustment.
· Ensure vehicle is secure and wheels are blocked.
· Shut off the engine, leaving the transmission in a
low gear or park then release the spring parking
brakes.
· Make a chalk mark where each push rod enters
the brake chamber.
· Reapply the spring parking brakes and measure
the distance from the brake chamber to the
chalk mark. Ensure the slack (push rod travel) is
within 3/4 and 1 1/2 inches or within
manufacturer’s specifications and the angle
between the slack adjuster and push rod is 90°
or as close as practical. If not, then a brake
adjustment is required.
Brake off
Brake on
Chalk mark
Chalk mark
Ruler
When the brakes are out of adjustment, braking
efficiency is reduced by three factors:
1. Brake lag increases because additional air is
required to fill and pressurize the increased
chamber volume caused by the increased stroke
of the push rod.
2. The angle between the slack adjuster arm and the
push rod becomes more than 90°, which results in
a loss of force between the linings and
the drum. (see diagram on page 18)
3. The effectiveness of diaphragm brake chambers
drops off significantly if the stroke exceeds 75% of
its designed travel. For a type 30 chamber (30
square inches of effective diaphragm area) that
has a design stroke of 2 1/2 inches, the brakes
should be adjusted at or before 1 1/2 inches of
travel. With a working pressure of 100 psi this
chamber will produce a force of 3,000 lb at 1 1/2
inches of travel, but only 2,500 lb of force at a
travel of 2 1/4 inches. Remember, when a chamber
bottoms out, the force reduces to zero.
66