Spring Brakes
release the brakes. Brake again as soon as the tires gain
traction.
All trucks, tractor-trailers, and buses are equipped
with emergency and parking brakes. These brakes are
applied by the mechanical force produced by a spring
brake. When you are driving, the powerful springs are
held back by air pressure. A leak in the air pressure
system causes the springs to engage the brakes.
Engaging the parking brake control on the dashboard
releases the air holding back the springs. This action
allows the springs to engage the brakes.
Tractor and straight truck spring brakes engage
when the air pressure drops to a pressure ranging
between 20 to 45 psi. Do not wait for the brakes to
engage automatically. When the low air pressure
warning light and buzzer activates, you should bring the
vehicle to a safe stop immediately, while you can still
control the brakes.
The braking power of the spring brakes depends on
the adjustment of the brakes. If the brake adjustment is
incorrect, the regular, emergency, and parking brakes
cannot work correctly.
Parking Brake Controls
On newer vehicles, a diamond shaped, yellow,
push-pull control knob is used to engage or disengage
the parking brakes. When the knob is pulled out, the
brakes are engaged. By pushing the knob in, you can
release the brakes. On older vehicles, the parking brake
may be controlled by a lever. You should engage the
parking brake anytime the vehicle is parked.
Using Air Brakes
When using air brakes, you should push the brake
pedal down and control the pressure so the vehicle
comes to a smooth, safe stop. If your vehicle is equipped
with a manual transmission, do not push the clutch in
until the engine rpm is lowered to idle speed. When
stopped, select a starting gear.
EMERGENCY STOPS. When applying air
brakes in an emergency stop, brake so you can steer and
keep your vehicle in a straight line. Use methods of
controlled or stab braking.
Controlled Braking. Controlled braking is also
called squeeze braking. Controlled braking is
applying the brakes as hard as possible without locking
the wheels. Do not turn the steering wheel while doing
this. If steering is required or the wheels begin to slide,
Stab Braking.The stab braking method requires
applying the brakes as hard as possible and releasing
them when the wheels lock up. As soon as the wheels
start to roll, apply the brakes fully again. It can take up
to 1 second for the wheels to start rolling after releasing
the brakes. You should stay off the brakes long enough
to get the wheels rolling again; otherwise, the vehicle
may not stay in a straight line.
DOWNHILL BRAKING. The correct method
for going down long grades is to use a low gear and
travel at a slow speed that allows a fairly light, steady
use of the brakes to prevent the vehicle from speeding
up. When you go slow and apply light pressure, the
brakes cool down and work properly.
CAUTION
When going down a hill, overuse of the
brakes can make them get too hot which causes
the brakes to fade. This can cause the operator
to press down harder on the pedal to gain the
required braking power. Prolonged action like
this can cause the brakes to fade until they stop
working.
Trailer Hand Valve
The trailer hand valve (also called the trolley valve)
controls the trailer brakes. Do not use it when driving
because of the danger of making the trailer skid. The
foot brake sends air to all of the brakes on the vehicle,
including the trailer. There is much less danger of
causing a skid or jackknife when only the foot brake is
used.
Tractor Protection Valve
The tractor protection valve keeps air in the tractor
or truck should the trailer break away or develop a bad
air leak. The valve is controlled by the trailer air supply
control valve in the cab of the tractor or truck. The
control valve allows the opening and shutting of the
tractor protection valve. The tractor protection valve
closes automatically when the air pressure drops to a
range of 20 to 45 psi. When the tractor protection valve
closes, it stops any air from escaping from the tractor. It
also shuts off the air from the trailer emergency line,
causing the trailer emergency brakes to engage.
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