with rear-wheel antilock brakes should step firmly with
care. and if they feel the wheel locking, they should
release some pressure.
Expect noise and vibration in the brake pedal
when antitock brakes are in use. The mechanical noise
or pulsation of antilock brakes when in use might catch
an operator by surprise; however, these sensations tell
you that the system is working.
Remember that you can steer while braking with
a four-wheel antilock brake system. Steering is not
always instinctive in an emergency. But steer out of
danger while braking with antilock brakes. And
remember that while you have steering capability, your
vehicle may not turn as quickly white braking on a
slippery road, as it would on dry pavement.
The rear-wheel antitock brakes typically found on
light-duty trucks provide vehicle stability but do not give
you the steering capability of four-wheel antilock brakes.
Anti lock brakes can often stop more quickly than
conventional brakes but they cant overcome the law of
physics. Antilock brakes function well on wet-paved
surfaces and icy or packed snow-covered roads.
Stopping times will be longer on gravel or fresh snow,
although operators wont experience the dangerous
lockup of wheels usually associated with conventional
Drive safely because antilock brakes are only as
good as the operators using them. Antilock brakes
cannot compensate for driving too fast, too aggressively
or failing to maintain a safe distance between vehicles.
They cannot guarantee recovery from a spin or skid
before braking. Also avoid extreme steering maneuvers
while antilock brakes are engaged.
Your antilock braking system instrument panel
tight will go on for a few seconds after starting the
ignition. The tight goes on so the system can conduct the
normal system test. If the tight does not go on during
ignition or if the tight goes on during normal driving,
this means that a problem has been detected and the
antilock braking system has been shut off. Conventional
braking will continue. Consult the manufacturers
service manual if this problem occurs.
Since exact antitock brake systems vary, consult
the vehicle manufacturers service and repair manuals
for more details of system operation.
Power brakes systems are designed to reduce the
effort required to depress the brake pedal when
stopping or holding a vehicle stationary. The booster is
located between the brake pedal linkage and the master
Most power brake systems use the difference
between intake manifold vacuum and atmospheric
pressure to develop the additional force required to
apply the brakes. When the operator depresses the
brake pedal, the power booster increases the amount of
pressure applied to the piston within the master
cylinder without the operator having to greatly
increase brake pedal pressure.
When a vehicle is powered by a diesel engine, the
absence of intake manifold vacuum requires the use of
an auxiliary vacuum pump. This pump may be driven
by the engine or by an electric motor.
On many modern vehicles, vacuum boosters are
used with the hydraulic brake system to provide easier
brake application. In a hydraulic brake system there
are limitations as to the size of the master cylinder and
wheel cylinders that can be practically employed.
Furthermore, the physical strength of the operator
limits the amount of force that can be applied, unless
the brakes are self-energizing. These factors restrict
the brake shoe to brake drum pressure obtainable.
Vacuum boosters increase braking force.
A vacuum booster consists of a round enclosed
housing and a diaphragm. The power brake vacuum
booster uses engine vacuum (or vacuum pump action on a
diesel engine) to apply the hydraulic brake system.
Vacuum boosters are classified into two types (fig.7-22)
atmospheric suspended and vacuum suspended. The
descriptions of the two types are as follows:
An atmospheric suspended brake booster (fig.
7-22) has normal air pressure on both sides of the
diaphragm when the brake pedal is released. As the
brakes are applied, a vacuum is formed in one side of the
booster. Atmospheric pressure then pushes on and
moves the diaphragm.
An vacuum suspended brake booster (fig. 7-22)
has vacuum on both sides of the diaphragm when the
brake pedal is released. Pushing down on the brake
pedal releases vacuum on one side of the booster. The
difference in air pressure pushes the diaphragm for
Air has a weight of approximately 15 pounds per
square inch at sea level. The weight of the air or
atmospheric pressure is what is used to operate the