A rubber boot prevents dust, dirt, and moisture
from entering the back of the master cylinder. The boot
fits over the master cylinder housing and the brake pedal
The reservoir carries a sufficient reserve of fluid
to allow for expansion and contraction of brake fluid
and brake lining wear. The reservoir is filled at the top
and is well sealed by a removable filler cap containing a
vent. Integral reservoirs are made of the same material
as the cylinder. whereas detachable reservoirs are made
The intake port or vent allows fluid to enter the
rear of the cylinder, as the piston moves forward. Fluid
flows out of the reservoir, through the intake port, and
into the area behind the piston and cup.
The compensating port releases extra pressure
when the piston returns to the released position. Fluid
can flow back into the reservoir through the
compensating port. The action of both ports keeps the
system full of fluid.
The residual pressure check valve maintains
residual fluid pressure of approximately 10 psi. This
pressure prevents fluid from seeping past the cups in the
wheel cylinders and also prevents air from entering the
hydraulic passages when the brakes are released.
Older vehicles used single piston, single reservoir
master cylinders that were dangerous. If a fluid leak
developed (cracked brake hose, seal damage, or line
rupture). a sudden loss of braking ability occurred.
Modern vehicles use dual master cylinders. These
master cylinders provide an additional safety feature in
that should one portion of the brake system fail. the
other system will allow the vehicle to maintain some
The dual master cylinder (fig. 7-5). also called a
tandem master cylinder, has two separate hydraulic
Figure 7-5.Dual master cylinder.