Air Brake System Components and Function
This domain tests how an air brake system produces, stores, controls, and applies compressed air to stop a vehicle. Oregon CDL questions focus on the compressor, governor, reservoirs, valves, brake chambers, slack adjusters, and foundation brakes - and on what each part does, not just its name.
Air production and storage
The engine-driven compressor pumps air into the supply reservoir. The governor controls compressor cut-out and cut-in pressures, usually about 120–125 psi cut-out and near 100 psi cut-in. A safety valve protects against overpressure, typically near 150 psi. Reservoirs store air; drain valves remove moisture and oil. Check valves stop air from flowing backward, so a failed circuit or disconnected trailer does not drain the whole system.
Control and application
The foot valve meters air to the brake chambers. Air pushes the diaphragm and push rod, which moves the slack adjuster and applies the foundation brake - an S-cam drum, wedge, or disc brake. When the pedal is released, air exhausts and the return spring releases the brake. Spring brakes are mechanically applied by a large spring and released by air pressure, so they set automatically when pressure is lost.
Worked example
A driver sees the gauge rise to about 120 psi, hears the compressor stop, then sees it restart near 100 psi. The governor is cycling the compressor correctly. The safety valve is not the control; it is a backup that opens only at excessive pressure, near 150 psi.
Traps that cost marks
Do not confuse the governor with the safety valve. Do not say the compressor creates hydraulic pressure. Do not treat the supply reservoir and service reservoirs as the same tank. Remember that spring brakes are applied by spring force, not by air pressure.