Disc Brake Assembly
Disc brakes are basically like the brakes on a
ten-speed bicycle. The friction elements are shaped
like pads and are squeezed inwards to clamp a rotating
disc or wheel. A disc brake assembly consists of a
caliper, brake pads, rotor, and related hardware (bolts,
clips, and springs), as shown in figure 7-16.
nonrotating unit in the system and it may be mounted to
the spindle or splash shield to provide support. The
brake caliper assembly includes the caliper housing,
the piston(s), the piston seal(s), the dust boot(s), the
brake pads or shoes, and the bleeder screw.
The caliper is fitted with one or more pistons that
are hydraulically actuated by the fluid pressure
developed in the system. When the brake pedal is
applied, brake fluid flows into the caliper cylinder. The
piston is then forced outward by fluid pressure to apply
the brake pads to the rotor.
The piston seal in the caliper cylinder prevents
pressure leakage between the piston and cylinder. The
piston seal also helps pull the piston back into the
cylinder when the brakes are released. The elastic
action of the seal acts as a spring to retract the piston
and maintain a clearance of approximately 0.005 inch
when the brakes are released.
The piston boot keeps road dirt and water off the
caliper piston and wall of the cylinder. The boot and
seal fit into grooves cut in the caliper cylinder and
p i s t o n .
A bleeder screw allows air to be removed from the
hydraulic system. It is threaded into the top or side of
Figure 7-16.Disc brake assembly.
the caliper housing. When loosened, system pressure is
used to force fluid and air out of the bleeder screw.
DISC BRAKE PADS.Disc brake pads consist
of steel shoes to which the lining is riveted or bonded.
Brake pad linings are made of either asbestos (asbestos
fiber filled) or semimetallic (metal particle filled)
friction material. Many new vehicles, especially those
with front-wheel drive, use semimetallic linings.
Semimetallic linings withstand higher operating
temperatures without losing their frictional properties.
Antirattle clips are frequently used to keep the
brake pads from vibrating and rattling. The clip snaps
onto the brake pad to produce a force fit in the caliper.
In some cases, an antirattle spring is used instead of a
A pad wear indicator (a metal tab) informs the
operator of worn brake pad linings. The wear indicator
produces an audible high-pitch squeak or squeal, as it
scrapes against the brake disc. This harsh noise is a
result of the linings wearing to a point, allowing the
indicator to rub against the brake disc, as the wheel
BRAKE DISC.Also called brake rotor, the
brake disc uses friction from the brake pads to slow or
stop the vehicle. Made of cast iron, the rotor may be an
integral part of the wheel hub. However, on many
front-wheel drive vehicles, the disc and hub are
The brake disc may be a ventilated rib or solid
type. The ventilated rib disc is hollow that allows
cooling air to circulate inside the disc.
Disc Brake Types
Disc brakes can be classified as floating, sliding,
and fixed caliper types. Floating and sliding are the
most common types. The fixed caliper may be found
on older vehicles.
FLOATING CALIPER.The floating caliper
type disc brake (fig. 7-17) is designed to move laterally
on its mount. This movement allows the caliper to
maintain a centered position with respect to the rotor.
This design also permits the braking force to be applied
equally to both sides of the rotor. The floating caliper
usually is a one-piece solid construction and uses a
single piston to develop the braking force.
Operation of a floating caliper is as follows:
Fluid under pressure enters the piston cavity and
forces the piston outward. As this happens the
brake pad contacts the rotor.