Braking ratio refers to the comparison of
front-wheel to rear-wheel braking effort. When a
vehicle stops, its weight tends to transfer to the front
wheels. The front tires are pressed against the road
with greater force. The rear tires lose some of their grip
on the road. As a result, the front wheels do more of the
braking than the rear.
For this reason, many vehicles have disc brakes on
the front and drum brakes on the rear. Disc brakes are
capable of producing more stopping effort than drum
brakes. If drum brakes are used on both the front and
rear wheels, the front shoe linings and drums typically
have a larger surface area.
Typically, front-wheel brakes handle 60 to 70
percent of the braking power. Rear wheels handle 30 to
40 percent of the braking. Front-wheel drive vehicles,
having even more weight on the front wheels, have
even a higher braking ratio at the front wheels.
The hydraulic system applies the brakes at all four
wheels with equalized pressure. It is pedal operated.
The system consists of the master cylinder, the wheel
cylinder, the brake lines and hoses, and the brake fluid.
The master cylinder is the primary unit in the brake
system that converts the force of the operator's foot
into fluid pressure to operate the wheel cylinders. It is
normally mounted to the firewall, which allows for
easy inspection and service, and is less prone to dirt
and water. The master cylinder has four basic functions
that are as follows:
It develops pressure, causing the wheel cylinder
pistons to move towards the drum or rotor.
After all of the shoes or pads produce sufficient
friction, the master cylinder assists in equalizing
the pressure required for braking.
It keeps the system full of fluid as the brake
It can maintain a slight pressure to keep
contaminants (air and water) from entering the
In its simplest form, a master cylinder consists of a
housing, a reservoir, a piston, a rubber cup, a return
spring, a rubber boot, and a residual pressure check
valve (fig. 7-4). There are two ports (inlet port and
compensating port) drilled between the cylinder and
reservoir. The description of the components of a
master cylinder is as follows:
The master cylinder housing is an aluminum or
iron casting having either an integral or detachable
reservoir. A cylinder is machined in the housing of the
master cylinder. The spring, the cups, and the metal
piston move within this cylinder.
The piston is a long spoonlike member with a
rubber secondary cup seal at the outer end and a rubber
primary cup at the inner end, which are used to
pressurize the brake system. The primary cup is held
against the end of the piston by the return spring. A steel
stop disc, held in the outer end of the cylinder by a
retainer spring, acts as a piston stop.
Figure 7-4.Cutaway view of a single master cylinder.