Air Brake System Components and Operation
This domain tests whether you can trace compressed air from the compressor to the brake chamber and say what each part does. Massachusetts questions here are about function and identification, not repair.
The air path
- The engine-driven air compressor pumps air into the storage tanks; it is cooled and lubricated by the engine and driven by a belt or gears.
- The governor watches tank pressure and controls the compressor. On a typical system it cuts the compressor out (unloads it) at about 125 psi and cuts it back in at about 100 psi.
- Air passes through a wet tank, which collects moisture and oil, before reaching the primary and secondary tanks. Each tank has a manual drain valve.
- A safety valve, usually set at 150 psi, protects the tanks if the governor fails and pressure keeps climbing.
- The foot valve (brake pedal) meters air to the brake chambers. Relay and quick-release valves speed application and release on long vehicles.
Service brakes vs. spring brakes
Service brakes are applied by air pressure and released by a return spring when the air exhausts. Spring (parking) brakes work the opposite way: a heavy spring applies them and air pressure holds them off, which is why they set automatically when pressure is lost.
Slack adjusters and foundation brakes
The pushrod moves the slack adjuster, which rotates the S-cam and forces the linings against the drum. An automatic slack adjuster keeps stroke correct; a manual one must be readjusted.
Worked example
You apply the brakes and measure 2 1/4 inches of pushrod travel on a Type 30 chamber rated for a 2 1/2-inch stroke. The limit for that chamber is 2 inches, so the brake is out of adjustment. The vehicle is out of service until the adjuster is set and the stroke re-measured.
Traps
Do not confuse which brake is applied by air (service) and which by spring (parking/emergency). Do not say the governor "makes" air, and do not describe the safety valve as the device that cycles the compressor.