Inspecting the Air Brake System
The Inspection domain asks you to sequence the air brake check correctly and to name what each step proves. Order matters as much as the numbers.
The three-part air brake check (engine off, key on)
- Leak check, brakes released. Build full pressure, shut the engine off, chock the wheels, release the parking brake, and watch the gauge for one minute.
- Leak check, brakes applied. Press and hold the treadle fully for one minute to test the applied system and chamber seals.
- Warning device and spring brake check. Fan the pressure down with the pedal. The lamp and buzzer must come on before 60 psi; the spring brakes must set in the 20–45 psi range, which you will hear and see at the knob.
Other pre-trip items
- Drain the tanks and look for water and oil.
- Compressor build-up: 85 to 100 psi within about 45 seconds at fast idle for a belt-driven compressor.
- Hoses and chambers: cracks, chafing, missing clamps.
- Slack adjuster pushrod travel within the manufacturer's limit, checked with the brakes applied.
Worked example
A candidate runs the warning test first on a system sitting at 40 psi. The buzzer sounds immediately, but that proves nothing because low pressure was already present. The valid order charges the system, checks leaks while it is quiet, then lets pressure fall to see where the warning actually triggers.
Traps
Leaving the engine running during the leak test, which masks leaks; forgetting to chock the wheels and release the parking brake; starting the warning test with a discharged system; checking pushrod travel with no brake applied.