The parking brake of the tractor unit and the emergency brake of the trailer are spring brakes that require air pressure in order to be released. They are applied when air pressure is released from the system, and disengaged when air pressure is supplied. This is a fail-safe design feature which ensures that if air pressure to either unit is lost, the vehicle will stop to a grinding halt, instead of continuing without brakes and becoming uncontrollable. The trailer controls are coupled to the tractor through two "gladhand connectors", which provide air pressure, and an electrical cable, which provides power to the lights and any specialized features of the trailer.