The operation of a dual system master cylinder isbasically the same as a single master cylinder. However,the dual system master cylinder has two separatehydraulic pressure systems. One of the hydraulicsystems normally is connected to the front brakes andthe other system to the rear brakes. If either the front orrear hydraulic system fails, the other system remainsoperational.The master cylinder, like other parts in the brakesystem, is subject to wear, leaks, and deposits orcorrosion on the cylinder wall and piston. Part of yourprestart operation is to check the cylinder reservoir fluidlevel and add clean brake fluid to maintain themanufacturer’s specifications.The brake lines transmit fluid and pressure from themaster cylinder to the wheel cylinders, which aremounted on the brake-backing plate, and change thehydraulic pressure into mechanical force. Inside eachcylinder are two pistons that move in opposite directionsby hydraulic pressure which pushes the brake shoesagainst the brake drum or disc. The brake shoes are madeof steel that transmits force to the lining which isattached to the face of the shoe and makes contact withthe brake drums or discs. During contact with oneanother, the lining and the drum or disc create thefrictional surface that gives the braking effect.AIR BRAKE SYSTEMAn air brake system uses compressed air to applythe brakes. Air under pressure can be convenientlystored and carried through lines or tubes. Considerableforce is available for braking since operating airpressure may be as high as 100 psi. All brakes on avehicle and on a trailer (when one is used) are operatedtogether by a brake valve. This valve and the relativelocation of most of the basic assemblies of an air brakesystem are shown in figure 3-44.Air CompressorThe air compressor pumps air into the air storagetanks (reservoirs). The air compressor is driven by theengine through gears or a V-belt. The compressor maybe air-cooled or may be cooled by the engine lubricationsystem. It may have its own oil supply or be lubricatedby engine oil. If the compressor has its own oil supply,the oil should be checked during your prestartoperations.GovernorThe governor controls the air compressor output.When air tank pressures rise to the cutout level at aboutFigure 3-44.—Typical air brake system.3-24
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