At what air pressure does the low-pressure warning device come on?

The low air pressure warning device must come on before pressure in either dual brake system drops below 60 psi. In most vehicles it activates at 55–60 psi. The warning may be a buzzer, a flashing light, a wig-wag arm that drops into view, or some combination of these. When the warning comes on while driving, stop and park as soon as it is safe to do so — air pressure has dropped to a level where the brakes are losing effectiveness and the spring brakes are not far from automatic application. Continuing to drive risks an unplanned brake application or a complete loss of braking. Spring brakes apply automatically below 20–45 psi, but the warning at 60 psi gives the driver time to stop safely before reaching that point.