Air Brake System Components and Operation
The Air Brake System Components and Operation domain tests whether you can name the parts that make air brakes work and explain what each one does under normal conditions. Expect questions on the compressor and governor, the reservoirs, valves, foundation brakes, and the two pressure gauges.
The air supply side
The engine-driven compressor charges the system through a governor that cuts the compressor out at roughly 120-125 psi and back in at about 100 psi. Air passes through an air dryer (or an alcohol evaporator in cold climates) into the wet tank, then to the primary and secondary reservoirs of the dual circuit. A safety valve protects against over-pressurization, opening at about 150 psi. The supply gauge shows reservoir pressure; the application gauge shows the pressure you are applying with the foot valve.
The delivery side
Pressing the treadle (foot) valve sends a signal to relay valves, which release stored air into the brake chambers. The pushrod moves the slack adjuster, which rotates the S-cam (or actuates a wedge or disc brake) and forces the linings against the drum. Spring brakes work the opposite way: a heavy spring applies them, and air pressure releases them.
Worked example
A Type 30 brake chamber has a 30-square-inch diaphragm. At 100 psi of application pressure, the pushrod force is 30 x 100 = 3,000 pounds before mechanical losses. That is why losing even 20 psi noticeably weakens braking: the same chamber at 80 psi produces only 2,400 pounds.
Traps that cost marks
- Mixing up the supply gauge (tank pressure) with the application gauge (brake pressure).
- Saying air applies the spring brakes; air releases them.
- Confusing the governor (cuts the compressor in and out) with the safety valve (protects at about 150 psi).