Air Brake Fundamentals
Air dryers, governors, brake chambers, slack adjusters, and foundation brake components.
- Identify the components of the air supply system and describe their functions
- Explain the operation of foundation brake components including drums, shoes, and cam shafts
- Describe how automatic slack adjusters maintain proper brake adjustment
Leçon 1
Air Supply System Components
How Air Brakes Generate Stopping Power
Commercial trucks and trailers rely on compressed air to apply brakes. Unlike passenger vehicles that use hydraulic fluid, heavy trucks use air pressure because it is available in unlimited supply and a small leak does not immediately cause total brake failure. Understanding the air supply system is fundamental to every aspect of truck and transport mechanic work.
The air supply system begins with the air compressor, which is gear-driven or belt-driven by the engine. The compressor draws in filtered air and pressurizes it, typically to a maximum system pressure controlled by the governor. The governor controls the air pressure in the primary and secondary brake circuits by cycling the compressor between its cut-in and cut-out pressures.
The Air Dryer
Compressed air contains moisture that can cause corrosion and freeze in cold weather, jamming valves and rendering brakes inoperative. The air dryer removes moisture from compressed air before it enters the wet tank. Most modern air dryers use a desiccant cartridge that absorbs moisture during the charging cycle and purges it during the governor cut-out cycle.
The air dryer is mounted between the compressor and the first reservoir (wet tank). When the governor signals cut-out, the air dryer purge valve opens, and a burst of dry air from the system flows backward through the desiccant bed, carrying accumulated moisture out the exhaust port.
Reservoirs and Protection Valves
After the air dryer, compressed air flows to the wet tank (supply reservoir), then through a dual circuit protection valve that splits the air supply into the primary and secondary circuits. This valve ensures that if one circuit develops a leak, the other circuit retains enough pressure for partial braking.
The tractor protection valve protects the tractor air supply if the trailer separates or develops a major leak. When trailer air pressure drops below a set threshold, the tractor protection valve automatically closes, isolating the tractor's air supply from the trailer lines.
The air dryer removes moisture from compressed air, the governor controls system pressure (cut-out at 120-135 psi), and the tractor protection valve protects the tractor air supply if the trailer separates. These three components are the most commonly tested air supply topics on the Red Seal exam.