125 pounds per square inch (psi), the governor stops thecompressor from pumping air. When the tank pressurefalls to the cut-in pressure at about 100 psi, the governorallows the compressor to start pumping again.Air Storage TanksAir storage tanks (reservoirs) are used to holdcompressed air. The number and size of air tanks variesamong vehicles. The tanks hold enough air to allow thebrakes to be used several times, even if the compressorstops working.NOTE: Compressed air usually has some water andsome compressor oil in it which is bad for the air brakesystem. For example, the water can freeze in coldweather and cause brake failure. The water and oil tendto collect in the bottom of the air tank; therefore, eachair tank is equipped with a drain valve in the bottom.The two types of drain valves areas follows:1. The manual valve shown in figure 3-45 isoperated by turning it a quarter turn or by pulling a cable.Part of your post-operational procedures is to drain allair tanks at the end of each day.2. The automatic valve automatically expels thewater and oil. This system may also be equipped with amanual drain.Alcohol EvaporatorSome air brake systems have an alcohol evaporatorto put alcohol into the air system. This helps reduce therisk of ice in air brake valves and other parts during coldweather. Ice inside a brake system can make the brakesstop working.Figure 3-45.-Air tank manual drain valve.If your vehicle has an alcohol system, the containershould be checked during each prestart operation andfilled up as necessary. Daily draining of the air tanks isstill required to get rid of the water and oil.Safety ValveA safety relief valve is installed in the first tank intowhich the air compressor pumps air. The safety valveprotects the tank and the rest of the system from toomuch pressure. The valve is usually set to open at 150psi. If the safety valve has to release air pressure,something is wrong in the air brake system. This shouldbe documented to inform the mechanic inspectors.Brake PedalThe brakes are applied by depressing the brakepedal (also called the foot valve, or treadle valve)that gives the operator control of the air brakesystem. When the brake pedal is engaged, air fromthe air tanks flows through the brake pedal valvethrough the brake lines to the brake chambers close tothe wheel brakes that contain flexible diaphragms. Theforce of the air admitted into these chambers causes thediaphragms to operate the brake shoes through amechanical linkage.Pushing the pedal down harder applies more airpressure. Letting up on the brake pedal reduces the airpressure and releases the brakes. Releasing the brakesallows some compressed air out of the system; therefore,the air pressure in the tanks is reduced and it must berecharged by the air compressor. Pressing and releasingthe pedal unnecessarily may release air out faster thanthe compressor can replace it, and should the pressurebecome too low, the brakes cannot work properly andbrake failure will occur.Pressure GaugeAn air pressure gauge lets you know if you haveproper air pressure within the reservoir. A low airwarning device should cut on before the pressuredrops to less than 60 psi in the air tank. This gauge isusually on the instrument panel of a truck or bus. If thepressure fails to buildup or exceeds the maximum limitsafter building up, secure the truck until the fault iscorrected.Hand Brake ValveIndependent control of brakes is necessary underbad conditions, especially if you have to put on the3-25
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