Air Brake System Components and Their Function
The chain you must be able to recite
Every air brake system stores energy, controls it and applies it. Learn the sequence: the compressor makes air; the governor loads and unloads the compressor between cut-in and cut-out; the reservoirs (wet tank plus primary and secondary dries) store it; the foot valve or treadle meters it; the relay valve at each axle delivers a large volume of air to the brake chambers; the chamber pushrod moves the slack adjuster, which rotates the S-cam, forcing the shoes against the drum. Valves and fittings you must recognise: low-pressure warning buzzer and light, one-way check valves, safety valve, drain cocks, quick-release valve, tractor protection valve, glad hands, and the dash application gauge.
Service versus spring chambers
The most heavily tested distinction: a service chamber applies when air enters and releases when air exhausts. A spring brake chamber applies by heavy spring force and releases only when air pressure compresses that spring. This is exactly why losing air pressure applies the parking brakes instead of releasing them.
Traps that show up in questions
Assuming the treadle feeds air directly to the chambers - on most heavy vehicles it sends a control signal, and the relay valve does the heavy delivery, which shortens brake lag. Confusing the quick-release valve (dumps service air fast so brakes release quickly) with the relay valve. Believing the governor builds pressure; it only switches the compressor between load and unload. Treating a low-pressure warning as a gauge fault when it is a system fault signal.
Rule of thumb with a worked example
Air applies the service brake; a spring applies the parking brake; air releases it. If a question says pressure falls to 30 psi, ask which way the spring goes - it extends and applies, so the vehicle stops and cannot be moved until pressure is restored and the push-pull valve is reset.