A low warning switch is installed after the supply reservoir to alert the driver when air pressure drops below a safe level 60 psi (414 kPa). The switch activates either (or a combination of) a buzzer, warning light or a wig-wag (a “flag” that drops into the driver’s view). If the low warning system activates, the driver must stop and determine the cause. The warning light must be operational. The buzzer/wig-wag are optional.