How long should the service brake be pumped during the low air warning test?

The correct procedure for the low air warning test requires pumping the service brake until the gauge stabilizes. This method ensures that the driver can accurately assess the function of the low air pressure warning system. When the brake is pumped during this test, the purpose is to see at what pressure the warning indicator activates, and allowing the gauge to stabilize provides a clear indication of whether or not the system is functioning as intended. Stabilizing the gauge confirms that the air pressure is dropping to a level that will trigger the warning light or alarm, thereby signaling a potential need for immediate attention before operating the vehicle. This is crucial for ensuring the safety and reliability of the braking system.