Trailer backfeeding at Tractor protection valve when applying service brakes.

QR valve is bad.

 

Bad/sticking Quick Release Valve (QR). The valve should expel air (exhaust) after the service brake is released.

 

You don’t have the glad hands on backwards do you….?

Don’t ask how I know this……

 

Nothing on the trailer can make that QR leak, if it's doing it with service brakes applied.

Different circumstance if only with brakes released, and no service applied. But in your case, that QR valve is the only option.

 

I agree with Big Road Skateboard on posts #2 and #6.
This from Bendix: When air pressure enters the supply port, it causes the center portion of the diaphragm to seal the exhaust in the body. Supply air pressure simultaneously forces the edges of the diaphragm away from its seat on the cap, allowing air to flow freely out the delivery port. Delivery pressure is the same as supply pressure.
The exhaust is not sealing properly and could be somewhat intermittent as in seals sometimes and some time not which is why switching trailers 'appeared' to solve the problem.
I can't even see a restriction after the QR valve causing the problem you are having as pressure in the valve causes the exhaust to seal (if the valve is working properly)
Valve is inexpensive and easy to change.
One of the first things anyone should learn about air brakes is if a valve is leaking do not automatically condemn the leaking valve. This is NOT one of those cases like say if an emergency/parking brake side pancake is leaking and back-feeding then leaking through a relay valve.

OPERATIONAL AND LEAKAGE TESTS 1. Block the vehicle’s wheels and fully charge the air system. 2. Apply and release air pressure rapidly to the QR-L™ valve several times and check for prompt response. 3. With air pressure applied to the supply port, apply a soap solution to the QR-L™ valve exhaust ports. Leakage of a 1” bubble in 5 seconds is permissible. 4. With the brakes still applied to the supply port, apply a soap solution around the valve where the cap and body are joined. No leakage at this point is permitted. If the valve does not function as described, or if leakage is excessive, replace it at any authorized parts outlet

 

Yes, if that QR is on the service side, and only leaks with brakes applied, that QR is bad.