Trailer backfeeding at Tractor protection valve when applying service brakes.

It’s not a brake chamber. I’ve diagnosed, found and changed plenty of brake chambers. I’ve found 2 on Trailers service side just recently on different Trailers. We haul light, the leaks only show up when brakes are applied hard. I release the brakes, apply the trolly, holding it with a bungee cord. Then walk back and find the leaks if any. Because the Trolly is spring loaded, apparently no one ever does that. I’ve always checked my Trailers brake chambers that way. Can’t hear the leak from the cab with engine running. Even though it’s an obviously loud leak, easy to find, doesn’t even move the dash pressure gauge. So the leaks get overlooked.