Air leak with trailer hooked up

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Hello I have a 2005 Peterbilt 379. When I apply the brake pedal air leaks out of the valve that is under the right rear corner of the cab. This only happens when it is hooked to a trailer and it doesn’t matter which trailer( I’ve hooked it to 3 different trailers with same results) I’ve been looking for airline leaks or anything else obvious. Any help diagnosing what is causing this would be greatly appreciated. Thank you!
 
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