How do you check that the air pressure warning signal activates correctly?

To test the low-pressure warning: with the engine off and the key in the on position, pump the brake pedal repeatedly to bleed air out of the system. Watch the air pressure gauge. The warning signal (buzzer, light, or wig-wag) must come on before pressure drops below 60 psi. If it does not, the warning system is defective and the vehicle must not be driven. This is one of the most important pre-trip checks because the warning is the driver's only notice that the air system is failing — a warning system that doesn't work means a brake failure could occur without warning. Continue pumping after the warning activates to check that the spring brakes apply automatically (typically at 20–45 psi).