ve h i c l e . T h e c a l i p e r a s s e m b l y, w h i c h r e m a i n s
stationary, attaches to the steering knuckle on the front
wheels or to the rear axle housing on the back wheels.
The disc is usually solid when used on a
lightweight vehicle and vented (for cooling) on a heavy
vehicle. Both sides of the disc are machined to provide
a smooth friction surface. The caliper contains one or
more hydraulic pistons, which cause the brake shoes
(one on either side of the disc) to squeeze the disc in a
Figure 2-28 illustrates the operation of a typical
disc brake assembly. In actual application, the brake
shoes (friction pads) are held in light contact with the
disc when the brakes are released by a piston return
Some disc brake shoes have telltale tabs that
contact the disc when lining wear has reached a
predetermined point. This results in a scraping noise
when the vehicle is operated, warning the operator that
Figure 2-24.--Brake shoe self-energizing action.
the brake shoes are badly worn and should be replaced.
Disc brakes require no adjustment. However, you
will occasionally have to add brake fluid to the master
cylinder reservoir, which supplies fluid for the disc
portion of the braking system. This is necessary
because the piston return springs, by keeping the shoes
against the disc as wear occurs, create a larger cavity
for the fluid in the caliper assembly. A booster
assembly is often used in disc brake systems, as they
have no self-energizing feature. Some features of the
disc brake make it more desirable than the drum brake;
namely, braking action is instantaneous when pressure
is applied to the caliper assembly, fading caused by the
heat is eliminated, and the brakes are not affected when
water is splashed onto the disc and linings.
Figure 2-25.--Front wheel brake assembly.
SERVICING DISC BRAKES.--Anytime a
This method of anchoring allows the movement of
vehicle with disc brakes is scheduled for maintenance,
the shoes necessary to expand against the drum when
the disc should be inspected for scoring and hard spots.
the shoes are forced against the drum, and the
Slight scoring results from normal braking. A disc that
s e l f - e n e rg i z i n g a c t i o n o f t h e p r i m a r y s h o e i s
is scored less than 0.015 inches can be used without
transmitted through the pivot to the secondary shoe
machining if the overall thickness of the disc is still
(fig. 2-26). Both shoes tend to revolve with the drum
within the manufacturer's specifications. Heavy
and wedge against the drum through the one anchor
scoring or hard spots require the disc to be machined.
pin. The other anchor pin causes a similar action when
The rust ridges that build up as wear occurs are of
the wheel is revolving in the opposite direction.
no concern unless new shoes are to be installed.
Placing new shoes on a disc that has rust ridges causes
the shoes to seat on the ridge, resulting in poor braking.
These ridges should be removed by grinding or
The disc brake assembly (fig. 2-27) consists of a
machining prior to installing new shoes. A special
metal disc and caliper assembly. The disc, which is
lathe is normally used for machining discs. However, if
bolted to the wheel hub, rotates with the wheel of the