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

So you started with gauge at 125 or so, went down to 50. Hmmmm Did you have Truck running, Tractor parking brake released, wheels chocked, (Trailer applied) Foot pedal applied hard, then shut it off. Holding foot pedal, gauge drops, has to be going somewhere. Gonna have to follow every air line. Hitting them with soapy water, every relay valve, Governor,check exhaust port isn’t leaking, or leaking around the adjustment screw, air dryer lines and spitter isn’t seeping air, check air bags around the bases, air lines to seats. Valve at back of cab for Trailer air lines. (Tractor protection valve). The relay valve for the front brakes is usually under the radiator, The rear brake relay is usually under the fifth wheel. They have an exhaust port on the bottom, with a round flat rubber dust cover washer, maybe leaking from there. Try wiggling the plastic lines underneath, maybe ones chafing, and is up against another, barely leaking till disturbed. Little air lines to cab air bags, leveling valves and the cab and suspension valves themselves.

 

I can hear an air leak if it takes all night to get down to 50 from 120.
You cant hear a leak that basically grenades its self soon as you turn off truck? Maybe throw the old lady under the truck and you go in the cab

 

Lol

 

What about the air lines to the trans you Changed a while back. Still looking good?

 

Like what was said before. Go around with a soapy mix in a spray bottle and spray every air line , fitting, air bags everything with a leak that bad it can't be hard to locate. I had a big air leak before shut the truck off and just followed the hiss, it was a valve under the dump body, found the leak in about 10 seconds no spray bottle needed for that leak.

 

I had a slow leak I couldn’t find. One day while fueling in the rain, I did a walk around, checking tires, and heard a squeaky leaking noise. The water made it very noticeable. A loose Trailer line coupling connection.

 

While I was fixing that valve under the dump body l sprayed the airbags for the hell of it and found a pinhole leak and I'm talking a pinhole. It's not that hard to find leaks. Especially a big one.