Air Brake System Components and Function
The Components and Function domain asks you to identify every part that makes a heavy-vehicle air brake work and to say what happens when one of them fails. Questions typically name a part, describe a symptom, and ask you to connect the two.
The supply side
- Compressor - engine-driven pump that makes the air. It has no idea when to stop.
- Governor - the switch that tells the compressor to load and unload. Typical cut-out is about 125 psi, cut-in about 100 psi, and cut-out must not exceed 135 psi.
- Reservoirs - store air; each circuit has its own. Drain daily to remove water and oil.
- Check valve - one-way valve that stops air flowing backward out of a tank, so one leak cannot drain the whole system.
- Safety valve - spring-loaded pop-off set near 150 psi, protecting the tank if the governor fails.
The control and apply side
- Treadle (foot) valve - meters air to the brake chambers in proportion to pedal travel.
- Brake chamber - converts air pressure into pushrod stroke.
- Slack adjuster and S-cam - the adjuster transfers and multiplies the stroke; the S-cam spreads the shoes against the drum.
- Low-pressure switch - triggers the dash lamp and buzzer.
Worked example
A gauge climbs past 135 psi and the safety valve never pops. The governor has failed to unload the compressor, and the safety valve should then have relieved at about 150 psi, so it is stuck too. Answer "what controls normal cycling" with the governor; answer "what limits maximum pressure" with the safety valve.
Traps
Swapping governor and safety valve; saying the compressor "holds" pressure in the tank; forgetting the check valve's role; assuming the foot valve regulates tank pressure.