Trailer protection valve leaking with one of the trailers only

My tractor is leaking out the exhaust port when one of the trailers is hooked up during when performing an applied leakage test (truck in gear engine off, valves in, service brake applied).

I hear absolutely no air loss anywhere but the exhaust port on the Bendix TP5, and it leaks a good amount.

However, with my other trailer, the tractor performs the test perfect with no leaks.

What would cause this? I am confused since I found no leaks on the trailer..

Thank you

 

Changed both cans, nada. I should have tested first but the cans looked old and were cheap enough to replace as preventive maintenance.

It ended up being the relay valve.

Thank you!