mbware

I have a 5 ton dump truck where all the brake componets have been replaced. Master cylinder, hydravac, wheel cylinders etc. Brake master has more than required 1/2 inch slack. two new hydravacs have been installed. Around the field the brakes work perfectly. stops on a dime and releases pressure properly. Put the vehicle on the road and travel variable distance of 7 to 20 miles at speeds of 45 mph or greater and the truck stops fine but retains air pressure on the brake light switch and the booster... thus the brakes are locked. Bleed master cylinder at bleed nipple without any release. Pressure must be released from outlet on booster to release. Anyone have a clue as to the problem? Compressor, pressure regulator and all else check out fine. Thanks for any clues