What is a 'stop light switch' in an air brake system?

The stop light switch is an electric switch operated by air pressure in the service brake line. When you press the brake pedal, air pressure rises in the service brake line, the switch closes, and the brake lights at the rear of the vehicle turn on. This is the same function as in a passenger car, but the trigger is air pressure instead of mechanical pedal movement. The brake lights are essential for communicating to drivers behind that you are slowing or stopping. During pre-trip inspection, check that brake lights come on when the pedal is pressed and turn off when it is released. A burned-out brake light or failed stop light switch is a serious safety problem.