Air Brake System Components and Functions
The pressure path
Learn the system as a chain of pressure: compressor, governor, reservoirs, foot valve, brake chambers, slack adjusters, foundation brakes. The compressor (engine-driven, belt or gear) builds pressure and is protected by an air dryer that removes moisture and oil before air reaches the tanks. The governor watches reservoir pressure and cuts the compressor in at about 100 psi and out at about 125 psi on a typical system; the cut-out must not exceed the tank's safe rating. Safety valves on the reservoirs protect against governor failure by venting near 150 psi.
Valves you must be able to name
- Foot (treadle) valve: the driver's pressure regulator - it applies air in proportion to pedal travel; it does not create pressure.
- Relay valve: mounted near the brakes it serves, it speeds application and release by sending reservoir air straight to the chambers instead of routing it all through the treadle.
- Quick-release valve: exhausts chamber air fast on release so brakes let go promptly.
- Double check valve: lets either of two supply sources (for example, service and parking circuits) feed a device without backflow.
- Spring (parking) brake chamber: spring applies, air releases; the pushrod extends when air is lost.
- Slack adjuster: converts chamber pushrod stroke into shoe movement; automatic adjusters keep stroke in range.
- Low-pressure warning: buzzer/light must activate before pressure falls below 60 psi.
Traps and a rule of thumb
Questions love confusing the governor (controls compressor cycling) with the safety valve (emergency vent), and the treadle valve (meters air) with the relay valve (boosts volume). Another trap: the spring brake applies when air is lost, so a leaking chamber can drag or lock a wheel. Rule of thumb: governor cut-in near 100 psi, cut-out near 125 psi, warning by 60 psi, safety valve near 150 psi.