Air Brake System

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