Air Brake System Components and How They Work
Supply side vs. control side
Every air brake question sorts into two halves. The supply side makes and stores air: the engine-driven compressor pumps air through an air dryer (or an alcohol evaporator on older trucks) into the supply (wet) reservoir, then through check valves into the primary and secondary reservoirs. The governor watches reservoir pressure and unloads the compressor at about 125 psi (cut-out), restarting it around 100 psi (cut-in). Drain cocks at the bottom of every tank remove moisture and oil, and the safety valve (typically set near 150 psi) protects the system from over-pressurization.
The control side spends that stored air. The foot (treadle) valve meters air out in proportion to pedal pressure. Relay valves speed application and release at the rear axles; a quick-release valve does the same at the front. Air enters the service brake chamber, pushes the diaphragm, and extends the push rod. The push rod turns the slack adjuster, which twists the S-cam (or wedge, or disc) to force the shoes against the drum or rotor.
Traps: exam items love to swap the safety valve and the low-pressure warning switch. The safety valve guards against too much pressure; the warning switch alerts you to too little. Also remember the spring brake chamber is the mirror image of the service chamber: air pressure releases it, and a spring applies it.
Rule of thumb: if a part never touches a wheel, it belongs to the supply side; if it moves the push rod, it is control side.