Trailer brakes not releasing

Well I'm suspecting a plugged or malfunctioning valve on the tractor. The service line only supplies a pilot signal to the trailer service brake relay. If you set truck brakes, release trailer brakes, then step on the brake pedal to get the trailer brakes to stick. Then break the blue SERVICE gladhand (not the red supply one). There should be no air in that line to escape. If air does escape, you have an air valve on the tractor that is either sticking or the exhaust port is plugged up.

 

Should put an inline quick release at the tractor protection valve like all the Kenworths have on the application port..
But you do have a sticking valve or kinked air line issue, just put some dummy glad hands on to cap off your air lines, charge the supply line then you can make some brake applications and find where the air is exhausting, that would most likely be your problem valve or pinched line between valve and TPV.

 

On the break pedal