AIR BRAKES - GENERAL INFORMATIONGENERAL INFORMATIONAir brake equipment on trucks and truck tractorsprovides a means of controlling brakes through amedium of compressed air. The air system consists ofvarious components required to maintain a supply of air,direct the flow of air and convert energy of compressedair into mechanical force to apply the brakes. Differenttypes and sizes of devices are used on different types ofvehicles to meet operating requirements.The National Highway Traffic Safety Administration(NHTSA), branch of the Department of Transportation(DOT), issued a special Federal Motor Vehicle SafetyStandard 121 (FMVSS 121) which specifies that allchassis equipped with air brakes must meet certainbrake performance requirements.Some factors that govern braking effort andeffectiveness are brake design, brake lining area, brakedimensions such as diameter and width, contactpressure and type of lining material (functionalcharacteristics). The law has special requirements foremergency performance which means if air loss occurson any one of a vehicle’s axles, the chassis mustdemonstrate stopping capability by activation of theservice brake control. All chassis with air brakes willhave dual or split air control circuits consisting of aprimary and secondary system.AIR SYSTEMThe air system is a split system consisting of a primaryservice system and secondary service system.This split system begins where the main air supplyreservoir branches off into two separate (primary andsecondary) systems. This takes place through a seriesof check valves, separate air reservoirs and a brakevalve which has two supply and delivery systems forservice and emergency braking. The purpose of thissplit system is to provide a means of making a controlledstop if a failure occurs in either the primary or secondaryair system. A tractor has the advantage of utilizing thetrailer air brake system during emergency stopping.There are three reservoirs in the split air system:Supply Air Reservoir (Wet Tank)Filled directly by the air compressor. Some air reservoirswill be a two or three compartment type.Primary Air ReservoirAir supply for the major portion of the split service brakesystem.Secondary Air ReservoirAir supply for the balance of the split system. Thisreservoir is also the air pressure supply for the brakeinversion system on straight trucks.INITIAL SPLIT AIR SYSTEMInitial FMVSS 121 split air system has the primaryservice brake system on all chassis at the rear axlebrakes and secondary service brake system at the frontaxle brakes. When the vehicle has tandem rear axles,the service brakes on both rear axles are in the primaryservice brake system. These are termed the "initial" splitair systems. The initial split air system has relay valves(Figure 1) in the primary system and some park brakesystems. Refer to Figures 3 through 7 for systemidentification. You will note that some initial systemsutilize a quick release valve at the front axle in thesecondary system.Figure 1 - Relay ValveFigure 2 - Quick Release ValveCTS-4154Y - Page 2
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