cylinder consists of a cylinder or housing, expander
spring, rubber cups, pistons, dust boots, and bleeder
screw (fig. 7-10).
The wheel cylinder housing encloses all the
other parts of the assembly. It has a precision cylinder in
it for the pistons, cups, and spring.
The expander spring assists in holding the rubber
cups against the pistons when the assembly is NOT
pressurized. Sometimes the end of the springs has metal
expanders (cup expanders) that help to press the outer
edges of the cups against the wall of the wheel cylinder.
The wheel cylinder cups are special rubber seals
that keeps fluid from leaking past the pistons. They fit in
the cylinder and against the pistons.
The wheel cylinder pistons transfer force out of
the wheel cylinder. These metal or plastic plungers act
on pushrods that are connected to or directly on the
The dust boots keep road dirt and water from
entering the cylinder. They snap into grooves that are
cast on the outside of the housing.
The bleeder screw provides a means of removing
air from the brake system. It threads into a hole in the
back of the wheel cylinder. When the screw is loosened,
hydraulic pressure is used to force air and fluid out of the
BRAKE SHOES.Brake shoes are used to
support, strengthen, and move the brake lining.
Because the brake lining material is soft and brittle, it is
necessary to add a supportive foundation to the lining
so it will not collapse and break during use. The brake
shoes also serve to attach the brake lining to a
stationary unit. usually the backing plate. so braking
action can be accomplished.
Figure 7-10.Cross section of a wheel cylinder.
Brake shoes are made of malleable iron, cast steel,
drop forged steel, pressed steel, or cast aluminum.
Pressed steel is the most common because it is cheaper
to produce in large quantities. Steel shoes expand at
about the same rate as the drum when heat is generated
by braking application, thereby maintaining the
correct clearance between the brake drum and brake
shoe under most conditions.
Automotive brake shoes consist of a primary and
secondary shoe. The primary brake shoe is the front
shoe and normally has a slightly shorter lining than the
secondary shoe. The secondary shoe is the rear shoe
and has the largest lining surface area.
Variation in brake design and operating conditions
makes it necessary to have different types of brake
linings. Brake linings come in woven and molded form
The molded form is currently used on modern
vehicles. Molded brake lining is made of dense,
compact, asbestos fibers, sometimes impregnated with
fine copper wire, and cut into sizes to match the brake
shoe. Depending on how much metal fiber is used in
their construction determines how they are classified,
either as nonmetallic, semimetallic, and metallic
Nonmetallic linings contain very few metal
fibers. This type of lining is used on many vehicles
because of its quiet operation and good heat transfer
qualities. Because of the lack of metal particles, the
nonmetallic linings wear well with brake drums and do
not tend to wear the drum excessively.
Semimetallic linings have some metal particles
in their composition. They also have good wearing
properties and are quiet during application.
Metallic linings have a high degree of metal fiber
in their construction and are generally characterized by
small pads bonded or welded to the brake shoe. The
pads may have a small space between them to aid in
cooling. The metallic linings operate at high
temperatures and may require the use of special
high-temperature brake parts. Metallic brake linings are
generally used for heavy-duty brake application where
large loads must be stopped or brakes are applied often.
Brake lining is riveted or bonded to the face of the
brake shoe. Semitubular brass rivets are used to attach
the lining to the shoe. Brass rivets are chosen over
other types because brass does not score the brake
drums excessively if the lining should be neglected and
worn past the point of replacement.