CDL Air Brakes
How do you test the low-air warning device?
Fully charge the air system, leave electrical power on, and repeatedly apply and release the service brake to lower pressure. Watch the gauges and identify when the visual and audible warning activates.
Short answer
Fan the brakes and call the warning before the required pressure boundary.
The exact boundary you must state can depend on the system, vehicle, and current state test procedure. Learn the sequence and confirm the parameter your licensing agency requires.
A practical recall sequence
- Charge: bring the air system to normal operating pressure.
- Secure: follow the current inspection setup for the vehicle and test.
- Power: shut the engine off when instructed and keep the electrical system on so the warning devices can operate.
- Reduce: rapidly apply and release the foot brake to lower air pressure.
- Call: state when the warning light, buzzer, or flag activates and compare it with the required parameter.
Official source
Check Section 5: Air Brakes.
Review the current California Commercial Driver Handbook, Section 5, then use FMCSA's directory to find your own state's current manual.