Braking Systems, Air Brakes and Auxiliary Systems
What the exam tests
This domain covers air-brake components, daily tests, brake fade, retarders, ABS and spring brakes. Questions often ask what a gauge reading or warning means and whether the vehicle may be driven.
Core rules
- Air system: compressor, governor, reservoirs, foot valve, relay valves, slack adjusters and brake chambers. The governor cuts out around 120 psi (827 kPa) and cuts in around 90 psi (620 kPa) on many systems; verify your vehicle's specification.
- Daily test: build pressure, check for abnormal leak-down with brakes released and applied, confirm the low-pressure warning activates before pressure falls too low (commonly around 60 psi/414 kPa), and test spring-brake application.
- With engine off and brakes released, a common CDL-style check allows no more than 3 psi/min loss for a single vehicle or 4 psi/min for a combination. Local standards may differ.
- Spring brakes apply automatically when air pressure is lost. Never rely on air alone to hold a parked vehicle; use the parking brake and chocks where required.
- Brake fade: continuous use overheats drums and reduces friction. Use a lower gear and retarder on descents.
- ABS is an anti-lock system; it helps maintain steering control but does not always shorten stopping distance.
Worked example
During the daily test the low-pressure warning does not sound as pressure falls below 60 psi. Reasoning: the warning circuit is part of the safety system and the vehicle must not be dispatched. The driver records the defect and reports it; the vehicle stays out of service until the warning is repaired and retested.
Traps
- Fanning the brake pedal on a long descent instead of using a retarder and lower gear.
- Skipping the leak-down test because the gauge "looks fine".
- Assuming ABS guarantees a shorter stop on gravel or ice.
- Not knowing that spring brakes apply when air pressure drops.