Learning Objective: Explain the hydraulic and
mechanical principles of a brake system. Describe and
define the major components of hydraulic, air, and
air-over-hydraulic brake systems. Explain the operation
of hydraulic, air, and air-over-hydraulic brake systems.
Summarize the operation of antilock braking systems.
The brake system is the most important system on
a vehicle from a safety standpoint. You, as the
mechanic, are trusted to do every service and repair
operation correctly. When working on a brake system,
always keep in mind that a brake system failure could
result in a fatal vehicle accident. It is up to you to make
sure the vehicle brake system is in perfect operating
condition before the vehicle leaves the shop.
Braking action is the use of a controlled force to
accomplish three basic tasksto slow down, stop, or
hold the wheels of a vehicle stationary. Braking action
is accomplished by rubbing two surfaces together that
cause friction and heat (fig. 7-1). Friction is the
resistance to relative motion between two surfaces in
contact. The brakes convert kinetic (moving) energy
into heat to stop the vehicle. Heat energy is an
unwanted product of friction and must be dissipated to
the surrounding environment as efficiently as possible.
HYDRAULIC BRAKE SYSTEM
Learning Objective: Describe the operation, terms,
and component functions of a hydraulic brake system.
Describe the procedures for servicing a hydraulic brake
In hydraulic braking systems, the pressure applied
at the brake pedal is transmitted to the brake
Figure 7-1.Development of friction and heat.
mechanism by a liquid. To understand how pressure is
transmitted by a hydraulic braking system, it is
necessary to understand the fundamentals of
hydraulics (refer to chapter 3 of this TRAMAN). There
are two common types of hydraulic brake systems used
on modern vehiclesdrum and disc brakes.
PRINCIPLES OF BRAKING
It is known that to increase the speed of a vehicle
requires an increase in the power output of the engine.
It is also true, although not so apparent, that an increase
in speed requires an increase in the braking action to
bring a vehicle to a stop (fig. 7-2). A moving vehicle,
just as any other moving body, has what is known as
kinetic energy. Kinetic energy is the energy an object
possesses due to its relative motion. This kinetic
energy, which increases with speed, must be overcome
by braking action. If the speed of the vehicle is
doubled, its kinetic energy is increased fourfold;
therefore, four times as much energy must be
overcome by the braking action.
Brakes must not only be capable of stopping a
vehicle but must stop in as short a distance as possible.
Because brakes are expected to decelerate a vehicle at
a faster rate than the engine can accelerate, they must
be able to control a greater power than that developed
Figure 7-2.Braking requirements.