2011 ProStar Air Brake issue

I have a 2011 Prostar that is leaking air thru the trailer service line causing the trailer brakes to drag while going down the road. The air leak only occurs when the trailer air supply is engeged(the red button is pushed in). I have isolated the issue to the tractor, the tractor bake chambers have been checked, replaced the red/yellow valve in the dash, and replaced the relay valve that sits under the fifth wheel. Any suggestions would be appreciated!

 

Thanks

 

Thanks Heavy

 

There is an air relay valve that sits directly below the driver under the cab. Dealer told me it was the Tractor Protection Valve. It is just a standard relay valve, but replacing it solved my problem .