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One of those relay valves may be for the trailer, not sure. You can follow brake tubing to be sure. I wish I had one to look at for you. as Im not sure how the foot valve operates on those. but another thought is, to test the foot valve, is...when locked up...remove the pilot line coming back...

Here is a test for a relay valve. With air pressure built up, and truck NOT running. WHEELS CHOCKED!!! Crawl under truck and find relay valve. Have a friend apply the brakes. When they release the pedal, there should be a release of air from the quick release valve. I have seen mud duabers...

I am not familar with the 923 brakes, but there may be a quick release or relay valve near the rear axle somewhere thats hung up. thats the way it is on commercial trucks. Caging the brake chambers will only "hold" back the parking brake spring from applying. not the service side as you...