Air Brake System Components and Functions
An air brake system is a network of parts that stores, moves, and applies compressed air to slow and stop the vehicle. The compressor, driven by the engine, builds pressure and sends air through the system. A governor monitors reservoir pressure and controls when the compressor loads and unloads, keeping pressure within a safe range. An air dryer removes moisture and contaminants before air reaches the reservoirs, and reservoirs (tanks) store the compressed air. Safety valves protect against over-pressurization, and drain valves let you release moisture and contamination.
Pressure reaches the brakes through lines and hoses. The brake pedal (foot valve) meters air to the brakes, and the relay valve speeds up application and release by routing reservoir air to the chambers. Slack adjusters transfer pushrod motion to the brake shoes, and brake chambers convert air pressure into mechanical force. Spring brakes apply when air pressure is lost, and the parking brake valve controls them.
A common exam trap is confusing the governor (controls compressor cycling) with the safety valve (relieves excess pressure). Another trap is thinking the compressor produces brake force directly; it only supplies air. Questions often ask which component removes moisture (the air dryer) versus which stores air (reservoirs).
Rule of thumb: the compressor builds pressure, the governor regulates it, the dryer cleans it, reservoirs store it, and valves deliver it to the chambers that apply the brakes.