The operation of a dual system master cylinder is
basically the same as a single master cylinder. However,
the dual system master cylinder has two separate
hydraulic pressure systems. One of the hydraulic
systems normally is connected to the front brakes and
the other system to the rear brakes. If either the front or
rear hydraulic system fails, the other system remains
operational.
The master cylinder, like other parts in the brake
system, is subject to wear, leaks, and deposits or
corrosion on the cylinder wall and piston. Part of your
prestart operation is to check the cylinder reservoir fluid
level and add clean brake fluid to maintain the
manufacturer’s specifications.
The brake lines transmit fluid and pressure from the
master cylinder to the wheel cylinders, which are
mounted on the brake-backing plate, and change the
hydraulic pressure into mechanical force. Inside each
cylinder are two pistons that move in opposite directions
by hydraulic pressure which pushes the brake shoes
against the brake drum or disc. The brake shoes are made
of steel that transmits force to the lining which is
attached to the face of the shoe and makes contact with
the brake drums or discs. During contact with one
another, the lining and the drum or disc create the
frictional surface that gives the braking effect.
AIR BRAKE SYSTEM
An air brake system uses compressed air to apply
the brakes. Air under pressure can be conveniently
stored and carried through lines or tubes. Considerable
force is available for braking since operating air
pressure may be as high as 100 psi. All brakes on a
vehicle and on a trailer (when one is used) are operated
together by a brake valve. This valve and the relative
location of most of the basic assemblies of an air brake
system are shown in figure 3-44.
Air Compressor
The air compressor pumps air into the air storage
tanks (reservoirs). The air compressor is driven by the
engine through gears or a V-belt. The compressor may
be air-cooled or may be cooled by the engine lubrication
system. It may have its own oil supply or be lubricated
by engine oil. If the compressor has its own oil supply,
the oil should be checked during your prestart
operations.
Governor
The governor controls the air compressor output.
When air tank pressures rise to the cutout level at about
Figure 3-44.—Typical air brake system.
3-24
