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Chapter 28. Truck Brake Systems. Objectives (1 of 3). Identify the components of a truck air brake system. Explain the operation of a dual-circuit air brake system. Understand what is meant by pneumatic and torque imbalance.
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Chapter 28 Truck Brake Systems
Objectives (1 of 3) • Identify the components of a truck air brake system. • Explain the operation of a dual-circuit air brake system. • Understand what is meant by pneumatic and torque imbalance. • Discuss the effects of the Federal Motor Vehicle Safety Standard No. 121 (FMVSS No. 121) on present-day air brake systems.
Objectives (2 of 3) • Identify the major components of an air compressor. • Describe the operation of desiccant and aftercooler-type air dryers. • Outline the operating principles of the valves and controls used in an air brake system. • Explain the operation of an air brake chamber. • Outline the functions of the hold-off and service circuits in truck and trailer brake systems.
Objectives (3 of 3) • Describe the operation of S-cam and wedge-actuated drum brakes. • Describe the operating principles of manual and automatic slack adjusters. • List the components and describe the operating principles of an air disc brake system. • Describe the major components and operation of parking and emergency braking systems. • Define the principles of brake balance.
Air Brake Sub-systems • The air supply circuit • Primary circuit • Secondary circuit • Dash control valves–parking/emergency circuit • Trailer circuit • Foundation brakes
Shop Talk • FMVSS No. 121 requires that governor cut-out be set within the range of 115 psi to 135 psi. The ATA TMC recommends that governor cut-out be set at 125 psi. • It is a legal requirement that governor cut-in occur at no more than 25 psi less than governor cut-out pressure. • If the difference between governor cut-out and cut-in is less than 20 psi, the result is too frequent cycling of the compressor loaded and unloaded cycles.
Shop Talk • Air dryers must be fitted with a safety valve. • Because the system safety valve is located in the supply tank, if the hose from the dryer to the supply tank got plugged (with ice or contaminants) or kinked, the governor would not cut out the compressor effective cycle, and the resulting high pressure could explode the air dryer.
Warning • Alcohol evaporators MUST be located downstream from the air dryer in the supply circuit. • Alcohol will turn air dryer desiccant into mush if pumped through the system.
Typical Air Reservoirs, Dump Valves,Check Valves, and Low Pressure Indicators
Shop Talk • Make sure you understand the function of each dash control valve in the system. • Block the wheels and get into a truck with a fully charged air system. • Use the controls and observe the effect.
Caution • When coupling a tractor to a trailer not equipped with spring brake chambers, always connect the gladhands and charge the trailer with air before attempting to couple the fifth wheel.
Shop Talk • One OEM survey of warranty returns indicates that over half of the foot valves submitted function perfectly. • This misdiagnosis results from a failure by the technician to understand how the valve operates normally. • Take some time to understand exactly how this important valve masters the operation of the service brakes.
Caution • If a tractor is converted to a straight truck, the bobtail proportioning valve must be replaced with a non-proportioning relay valve. • In a case in which a long-wheelbase tractor was converted to a wrecker, the operator complained of lazy service braking until the treadle was floored, when the brakes would lock. Replacement of the bobtail proportioning valve with a standard relay valve rectified the problem.
Caution • Because air is delivered to the trailer hold-off circuit before the trailer tanks are charged, the trailer can be moved before any service braking is possible. • You should charge trailer tanks before attempting to move the trailer.
Shop Talk • Current spring brake control valves must prioritize air to the hold-off circuit over air to the air tanks. • This means that when air builds up in the air system, it is first delivered to the hold-off circuit, enabling the release of the parking brakes. • This enables a vehicle with a disabled air supply system to be moved much more quickly. A much smaller volume of air has to be transferred to the vehicle before release of the parking brakes becomes possible.
Shop Talk • FMVSS No. 121 requires that the TP valve isolate the tractor air supply from that of the trailer when pressure drops to between 20 and 45 psi.
