Chapter 12 Disc Brake System Components and Operation 197
Figure 12-8. A—Illustrates brake pads that are new. Note the small amount of brake fluid behind the piston. B—The pads have worn
down, causing piston to travel farther out in its bore to maintain the correct rotor-to-pad clearance. There is now a larger volume of
brake fluid behind the piston. (Delco)
Small
amount
of fluid
Piston Pads
Caliper
A New lining B Worn lining
Large
amount
of fluid
Rotor
Low-drag calipers have greater clearance. This clear-
ance can be as much as .006” (.15 mm) when the brakes
are released. This is designed to remove all brake drag,
which aids in fuel economy. Calipers using this design
are always used with quick take-up master cylinders, dis-
cussed in Chapter 5.
Brake pad-to-rotor clearance adjustment is automatic.
As the pad linings wear, the piston moves further past the
seal. The seals can retract the piston enough to eliminate
brake drag, but no more. Therefore, as the pads wear, the
piston moves out further. Extra brake fluid enters the piston
chamber to compensate for piston travel. This is illustrated
in Figure 12-8.
Caliper Types
There are two methods of mounting disc brake
calipers, depending on the hydraulic piston arrangement.
Calipers are grouped as floating and fixed calipers. On
modern vehicles, floating calipers are the most common.
Fixed calipers are used on a few modern high performance
vehicles. The two types of calipers are discussed in the next
sections.
Floating Calipers
Floating calipers use one or two pistons, located
on the same side of the caliper. The caliper can move or
“float” back and forth in relation to the rotor. This floating
action is made possible by attaching the caliper in a way
that allows it to slide on the mounting hardware.
Floating Caliper Construction
Floating calipers are commonly made of cast iron or
aluminum. The most common method of allowing the cali-
per to slide is by the use of mounting bolts that allow the
caliper to move over them. This is shown in Figure 12-9.
These bolts are called guide pins or slider pins. To reduce
wear of the caliper metal, the guide pins contact bushings,
sometimes called sleeves. The actual movement between
the bolts and caliper is through these bushings.
On other vehicles, particularly light trucks and vans,
the sliding takes place between machined ways. A way is a
flat surface designed to allow smooth movement between
itself and another component that slides over it. Both the
caliper and the spindle mounting surfaces are machined
smooth so they can move against each other with a mini-
mum of friction. See Figure 12-10.
Caliper movement over the ways is controlled by
the use of supports, sometimes called keys. These sup-
ports may be held in place by a single mounting screw,
Figure 12-11. A leaf spring, usually referred to as a clip,
cuts down on caliper rattling. On other calipers, the
sliding action is controlled and damped by one-piece
combination metal and rubber bushings, Figure 12-12.
Some disc brake calipers have only one pin, and the
side away from the rotor is installed on a machined way,
Figure 12-13.
One and two piston calipers may be installed on
an integral cast extension of the spindle, as shown in
Figure 12-14. Other vehicles have a special adapter or
bracket, which bolts to the spindle. The caliper is then
bolted to the adapter. A caliper with a bolt-on adapter
is shown in Figure 12-15.
Floating Caliper Operation
One or two piston floating calipers operate in the same
manner. As the brakes are applied, hydraulic pressure builds
up in the cylinder bore behind the piston and their seals.
Pressure forces the piston outward. When the piston moves
enough to force the inner brake pad into contact with the
rotor, resistance to outward piston movement is increased. It
now becomes easier for hydraulic pressure to push the cali-
per backward instead of pushing further on the inner pad.