pistons and two fluid reservoirs. In the dual master
cylinder, the rear piston assembly is termed the
primary piston and the front piston is termed the
In some dual master cylinders, the individual
systems are designed where one master cylinder piston
operates the front brake assemblies and the other
operates the rear brake assemblies. This is knownI as a
longitudinally split system (fig. 7-6). A system that has
each master cylinder piston operating the brake
assembly on opposite corners of the vehicle is known a
diagonally split system (fig. 7-6). In either system, if
there is a leak, the other master cylinder system can
still provide braking action on two wheels.
When the systems are intact (no leaks), the pistons
produce and supply pressure to all four of the wheel
cylinders. However, if there is a pressure loss in the
primary circuit of the brake system (rear section of the
master cylinder), the primary piston slides forward and
pushes on the secondary piston. As shown in figure
7-5, this action forces the secondary piston forward
mechanically, building pressure in two of the wheel
cylinder assemblies. Should the secondary circuit fail,
braking for the other two wheels would still be
Figure 7-6.Dual master cylinder braking systems.
available. The secondary piston slides completely
forward in the cylinder, as shown in figure 7-5. Then
the primary piston provides hydraulic pressure to the
other two brake assemblies. It is very unlikely that both
systems will fail at the same time.
When performing maintenance on a dual master
cylinder, you may notice that the front reservoir is
larger than the rear. This is a longitudinally split
system. The larger reservoir is for disc brakes. The
larger reservoir is necessary because as the disc pads
wear, they move outward creating a larger cavity in the
caliper cylinder and fluid moves from the master
cylinder to fill the additional area. To allow this action
to occur, the front reservoir of a longitudinally split
system has no residual check valve. However, with a
diagonally split system both reservoirs are the same
size and the residual check valve for the rear brakes are
located in the tees that split the system front to rear.
A wheel cylinder (fig. 7-7) changes hydraulic
pressure into mechanical force that pushes the brake
shoes against the drums. Other than the standard wheel
cylinder, there are two other types that you may come
in contact withthe stepped wheel cylinder and the
single-piston wheel cylinder.
The stepped wheel cylinder (fig. 7-7) is used to
compensate for a faster rate of wear on the front shoe
than on the rear shoe because of the self-energizing
action of the brakes. This condition requires a stepped
wheel cylinder with two bore sizes.
The single-piston wheel cylinder (fig. 7-7) is
used when it is desired that both brake shoes be
independently self-energizing, especially on the front
wheels. With this design it is necessary to have two
wheel cylinders, one for each shoe. Each cylinder has a
single piston and is mounted on the opposite side of the
brake backing plate from the other cylinder. Such an
arrangement is shown in figure 7-8.
For further information on wheel
cylinders, refer to "Drum Brake Assemblies"
in this chapter.
Brake Lines and Hoses
Brake lines and hoses transmit fluid under pressure
from the master cylinder to the wheel cylinders. The
brake lines are made of double-wall steel tubing with
double-lap flares on their ends. Rubber brake hoses are