Vehicles equipped with air brakes illuminate the PRIMARY BRAKE lamp when the air pressure in the brake system drops below 410-480 KPa (60-70 psi).

A low air pressure switch controls the ground side of the PRIMARY BRAKE lamp in order to illuminate it during low air pressure. An audible warning comes on along with the lamp. Air brake vehicles are also equipped with an air pressure gauge to inform the driver of the exact air pressure. If the air pressure continues to drop the Park Brake automatically sets, and the PARK BRAKE lamp illuminates until the air pressure in the brake system is increased and the Park Brake is released.