Air Brake System Components and Fundamentals
Air Brake System Components and Fundamentals
This domain tests how air is produced, stored, controlled, and turned into braking force. Expect questions on the compressor, governor, reservoirs, valves, brake chambers, slack adjusters, and foundation brakes, plus the pressure ranges that matter in the cab.
Core rules
- The engine-driven compressor builds air; the governor controls cut-out (about 125 psi) and cut-in (about 100 psi).
- Reservoirs store air. A safety valve protects the tank at about 150 psi; check valves keep air from flowing backward; drain cocks remove water and oil.
- The foot valve controls service brakes. Air pushes a diaphragm or piston in the brake chamber, moving the pushrod and slack adjuster to apply the foundation brake (S-cam, wedge, or disc).
- Spring brakes are mechanical parking/emergency brakes held off by air pressure; they apply when air is lost.
- The low-pressure warning must activate before pressure falls below 60 psi.
Worked example
A truck builds from 85 to 100 psi in 40 seconds and cuts out at 125 psi. The compressor is acceptable because the build-up is within 45 seconds and cut-out is in the normal range. If the same truck took 70 seconds, the compressor or governor would need attention before the vehicle passes a pre-trip inspection.
Traps
Do not confuse the safety valve with the governor: the safety valve is a last-resort pressure relief, not a normal regulator. Do not call spring brakes “air brakes” that need air to apply-they apply when air is lost. Remember that the service brake and parking brake are separate systems.