Why Do Truck Brakes Apply When Air Pressure Is Lost?

truck air brakes use compressed air to apply the service brakes, while powerful springs provide another way to brake. the key is where the air pressure acts, and what happens when it falls. pressing the pedal controls airflow through the relay valve to the brake chambers. inside each service chamber, a diaphragm moves a pushrod, turning the s-cam and pressing the brake shoes against the drum. the spring brake works differently. air holds its spring compressed; releasing that air lets the spring apply the brake. this also explains the parking brake and the automatic braking shown when a trailer breaks away. follow the air through the compressor, dryer, separate reservoirs and control valves, and see why a service-line leak does not necessarily apply the spring brake immediately.