Air Brake System Components and Function
The components domain is the largest block of questions on the Maine CDL air brake test. It asks you to identify parts, explain what each does, and predict how the system behaves when one part fails.
The compressed-air path
The engine-driven compressor pumps air through an air dryer and one-way check valves into the wet tank, then into the dry tank(s). A governor senses reservoir pressure and unloads the compressor at roughly 120–135 psi (cut-out) and lets it resume near 85–100 psi (cut-in). The safety valve in the wet tank opens near 150 psi as a backup if the governor fails, and drain cocks let you expel moisture and oil.
Control components
The foot (treadle) valve meters air to the relay valve, which sends a large volume quickly to the brake chambers. The push rod moves the slack adjuster, which rotates the S-cam to spread the shoes against the drum. Quick-release valves speed release; ratio and inversion valves manage split and spring-brake circuits; the tractor protection valve and trailer hand valve control trailer air. Spring brakes apply mechanically when air pressure is lost - they are parking and emergency brakes, not service brakes.
Worked example
A driver shuts the engine off with 120 psi showing and returns to find 90 psi, with no audible leak. Reasoning: 120 psi after cut-out is normal, and a small drop is normal only within the leak limit (3 psi/min single, 4 psi/min combination). A 30 psi drop is far outside that, so a leak exists and the vehicle must be repaired before dispatch.
Traps
Do not confuse the governor (cycles the compressor) with the safety valve (last-resort relief). The compressor does not build pressure with the engine off. Spring brakes are not applied by air pressure.