Figure 7-9.Brake lines and hoses.
Steel lines seldom need replacing except in areas
where they rust from exposure to salt air or constant
high humidity. Flexible hoses should be inspected at
regular maintenance periods for any signs of cracking
or abrasion. Should the outer protective covering be
cracked or badly abraded, it should be replaced.
Brake fluid is a specially blended hydraulic fluid
that transfers pressure to the wheel cylinders or
calipers. Brake fluid is one of the most important
components of a brake system because it ties all of the
other components into a functioning unit.
Vehicle manufacturers recommend brake fluid
that meets or exceeds SAE (Society of Automotive
Engineers) and DOT (Department of Transportation)
B r a k e f l u i d m u s t h a v e t h e f o l l o w i n g
Low freezing point (not freeze during cold
Water tolerance (absorb moisture that collects in
Lubricate (reduce wear of pistons and cups)
Noncorrosive (not attack metal or rubber brake
Maintain correct viscosity (free flowing at all
High boiling point (remains liquid at the highest
system operating temperature)
Standard brake fluid (DOT 3) is composed chiefly
of equal parts of alcohol and castor oil. This
combination of fluids works well under normal
conditions but it easily boils and becomes a vapor
under heavy-duty applications. Standard fluid also
tends to separate when exposed to low temperatures.
The increasing requirements of brake fluid led to the
development of silicone brake fluid.
After many years of research and development, a
brake fluid that was acceptable under extreme
operating conditions was developed. This fluid
achieved low water pickup and good corrosion
protection. The fluid also provides good lubrication
qualities and rubber compatibility. Silicone brake fluid
has been used in most military vehicles since the end of
D R U M B R A K E S
There are many types of brake system designs in
use on modern vehicles. Regardless of the design, all
systems require the use of rotating and nonrotating
units. Each of these units houses one of the braking
surfaces, which, when forced together, produce the
friction for braking action. The rotating unit on many
motor vehicle wheel brakes consists of a drum that is
secured to and driven by the wheel. The nonrotating
unit consists of the brake shoes and linkage required to
applying the shoes to the drum.
Drum Brake Assemblies
Drum brakes have a large drum that surrounds the
brake shoes and hydraulic wheel cylinder. Drum brake
assemblies consist of a backing plate, wheel cylinder,
brake shoes and linings, retracting springs, hold-down
springs, brake drum, and adjusting mechanism.
BACKING PLATE.The backing plate holds
the brake shoes, springs (retracting and hold-down),
wheel cylinder, and other associated parts inside the
brake drum. It also assists in keeping road dirt and
water out of the brakes. The backing plate bolts to the
axle housing or spindle.
assembly uses master cylinder pressure to force the
brake shoes out against the brake drum. It is normally
bolted to the top of the backing plate. The wheel