Air Brake System Components and Functions
What the system does
A vehicle air brake system stores energy as compressed air and uses it to apply the brakes. Know the role of each major part, because questions love to swap functions.
- Compressor - engine-driven pump that builds air pressure. It does not store air.
- Governor - senses system pressure and controls the compressor, cutting it out near the top of the operating range (commonly around 120–125 psi) and cutting it back in near the bottom (commonly around 100 psi). Trap: the governor does not regulate pressure at the wheels.
- Air reservoirs (tanks) - store air. The supply tank feeds the system; the primary and secondary tanks feed the two brake circuits.
- Safety valve - a spring-loaded valve on the supply tank that vents air if pressure climbs too high (often set around 150 psi). It is a backup, not a normal-pressure regulator.
- Drain valves / petcocks - let you remove moisture and oil from tanks, normally at the low point of each tank.
- Foot valve (treadle) - the driver's application valve; it meters air to the brake chambers.
- Brake chamber - converts air pressure into mechanical force on the slack adjuster and pushrod.
- Slack adjuster - links the pushrod to the brake camshaft and converts pushrod travel into brake application; it also adjusts for lining wear.
- Double check valve - lets air from either circuit reach a device without backflow between circuits.
- Low pressure warning device - warns the driver before pressure falls into an unsafe range.
Traps
Questions often confuse the governor with the safety valve, or the compressor with the reservoir. Remember: compressor builds, tanks store, governor controls the compressor, safety valve protects the tank.
Rule of thumb
Ask "does this part make air, store air, control air, or use air?" - placing each component in one category eliminates most distractors.