Air Brake System Components and Functions
What this domain covers
Components and Functions tests whether you can identify each part of a heavy-vehicle air brake system, state its job, and predict the result of its failure, using the federal CDL air brake material Kentucky follows.
The air path
An engine-driven compressor supplies air. The governor senses reservoir pressure and controls the compressor, cutting it in at about 85 psi and out at about 100-125 psi, then letting it idle (unload). Air passes through a dryer that removes moisture and oil, then into the supply (wet) tank and on to the primary and secondary tanks. Check valves between tanks prevent backflow, so a leak in one reservoir does not drain another. A safety valve on the supply tank vents at about 150 psi to prevent over-pressurization.
The foot valve (treadle) meters air to the brake chambers in proportion to pedal force. Air pushes a diaphragm and pushrod; the pushrod moves the slack adjuster, which rotates the S-cam (or drives a wedge) to spread the shoes against the drum. Releasing the pedal exhausts the air, and return springs pull the shoes clear.
Worked example
The gauge climbs to 105 psi and holds while the compressor note changes to a lighter, unloaded sound. That is normal: 105 psi is within cut-out range, and the sound is the governor unloading the compressor. If pressure instead kept rising past 150 psi with air escaping, the safety valve would be relieving because the governor failed to unload - a defect to fix before dispatch.
Traps that cost marks
- Confusing the governor (cut-in/cut-out) with the safety valve (pressure relief).
- Thinking the compressor stays loaded; it should unload.
- Mixing up the wet/supply tank (first, most moisture) with primary and secondary.
- Forgetting the slack adjuster links pushrod to brake; long stroke means out of adjustment.