Air Brake System Components and Their Functions
The Maryland CDL air brake test asks you to name each part of the system and describe what it does when you press or release the pedal. Most questions here are functional: given a symptom, identify the component; given a component, identify its job in the air path.
The air path
Air is compressed by the engine-driven compressor, passes through the air dryer, and is stored in reservoirs. The foot (treadle) valve meters stored air to the brake chambers; the pushrod pushes the slack adjuster, which twists the S-cam and applies the foundation brake. Relay and quick-release valves speed application and release on long wheelbases and trailers.
Parts you must be able to name
- Governor: loads and unloads the compressor to hold reservoir pressure in its normal range. It does not vent air itself.
- Air dryer: removes water vapor and oil before the air reaches the tanks.
- Check valve: one-way flow; keeps air from backfeeding and isolates a failed circuit.
- Safety valve: on the reservoir, vents at about 150 psi to prevent tank rupture.
- Spring brake chamber: mechanically applied by a heavy spring; air pressure holds it off.
- Tractor protection valve: shuts off the tractor air supply to the trailer if the trailer breaks away or supply pressure drops.
Worked example
A tractor-trailer loses air only when the trailer is connected; the tractor alone holds pressure. Reasoning: the loss is downstream of the tractor protection valve, so inspect the trailer supply and emergency lines, the gladhands, and the trailer relay valve before suspecting the compressor.
Traps
Two traps cost marks. First, confusing the safety valve (vents at 150 psi) with the governor (cuts the compressor in and out at roughly 100 and 120–125 psi). Second, reversing the spring brake: air releases it, and loss of air applies it.