Losing air pressure when braking hard or coming to a stop.

Once had a truck that went by the shop about 16 times a day going to the land fill. While the driver was hard to understand he took care of the truck. Not a cry baby and when he said something was wrong there was a problem. We could find nothing wrong for about 2 weeks. The truck was in the shop for a PM and annual inspection. I did the brake checks myself. Everything was perfect. One hundred yards out of the shop the alarm came on and air was dropping rapidly. I turned the key off and left it in gear. Stopped and jumped out. Found a 30/30 dumping air. Built air back up, pushed in the button and went back to the shop. Pulled in the shop and it had no leak what so ever. That was on the parking side not the service but it is possible.
I do not thank air will normally flow through a leveling valve fast enough to shut down a truck. Open for correction.
Never say always and always say maybe. That is a once in 40 year deal. Can always be some other deal.

 

Well did you find the leak??

 

A common drive axle and trailer brake chamber.
Good luck.

 

What make and year of truck?

 

That sucks!

 

Have you tried coupling both trailer air lines together? Not sure, but I think it will stop a bad tractor protection valve from leaking. If that’s the problem.

 

Yes, but when hooked to trailer, theres extra passthru for the air via all the air line routing in the trailer.

Try what ridenrepair said, or just get someone to push on the brakes both lines disconnected and feel for air coming out the red or blue lines.

Connecting them together and having no or significantly reduced air loss creates a closed loop and tells you one of two things
1. Tractor protection valve has failed
2. Main air supply (brake release valve in cab) has failed