If the spring brakes do not release when you push the trailer air supply control, check the air line connections. On trailers without spring brakes, if the air lines were crossed, you can drive but will not have trailer brakes. Test the trailer brakes before driving, using the hand valve or by pulling the air supply control. Each trailer and converter dolly has one or more air tanks. Air tanks are filled by the emergency supply line from the tractor.
Air pressure is sent from the air tanks to the brakes by relay valves.
The pressure in the service line tells how much pressure the relay valves should send to the trailer brakes. Do not let water and oil build up in the air tanks. Drain air tanks of water and oil after each working day. Water can freeze in parts of the brake system making the brakes fail. Trailer Service, Parking and Emergency Brakes On trailers without spring brakes, the emergency brakes come on whenever the air pressure in the emergency line is lost.