Air Brake System Components and Operation
This domain tests whether you can name the system's parts, explain how air pressure applies and releases the brakes, and recall the pressure values built into the design.
Making and storing air
- An engine-driven compressor supplies the air. The governor cuts it in at about 100 psi and out at about 125 psi, so it cycles instead of running constantly.
- Air passes through an air dryer (replaceable filter, often heated) and frequently an alcohol evaporator that stops moisture from freezing.
- Air is stored in the wet (supply) tank, then flows through one-way check valves to the primary and secondary tanks, so one circuit cannot drain another. A safety valve opens at about 150 psi.
Applying and releasing the brakes
- The foot (treadle) valve meters air to the chambers, letting you apply gradually.
- Air pushes the diaphragm and push rod; the push rod turns the slack adjuster, which twists the S-cam and spreads the shoes against the drum. Release vents the air and a return spring retracts the shoes.
- Relay valves speed application at the rear axles on long-wheelbase and combination vehicles.
Spring brakes
- A heavy spring applies the brake; air pressure holds it released. Losing air applies the brake.
- A caging bolt compresses the spring mechanically for towing only. Never open a spring chamber - the spring is under lethal compression.
Worked example
A driver builds to cut-out and shuts the engine off. At 55 psi the warning sounds; near 40 psi the spring brakes set with a clunk. That is normal: the warning must come before 60 psi and the spring brakes apply somewhere in the 20–45 psi range. Nothing needs repair.
Traps
- Expecting the warning exactly at 60 psi, or the spring brakes to wait for 0 psi.
- Thinking the compressor runs continuously; the governor cycles it.
- Confusing the safety valve (about 150 psi) with the governor cut-out (about 125 psi).
- Thinking the yellow knob applies the brakes with air. It releases air so the spring can apply them.