Air brakes use compressed air to slow down heavy trucks. An engine-driven air compressor pumps air into storage tanks. When you press the brake pedal, air flows to the brake chambers on the axles, pushing a pushrod that turns a camshaft to press the brake linings against the drums. Unlike car hydraulic brakes, air brakes are a release-to-apply system for safety.
This design includes spring brakes for emergency parking. Strong steel springs hold the parking brakes locked. The system must maintain at least 60 PSI of air pressure to compress these springs and let the wheels turn. If you lose air pressure on the highway, the springs release and the brakes lock up automatically to stop the truck. Drivers must pass an air brakes exam to have this restriction removed from their CDL.