Trailer air supply line pressure? I have a supply problem

The big one with the ABS cables is the service valve. The one pictured is the emergency valve. Air goes through it to the tank, if it's sticking (one side) the brakes will still release.

 

If you can find where you are losing that air supply . you should be able to narrow down where the problem is at. It almost sounds as if there is a blockage somewhere. Maybe a piece of debris inside the lines or a kink. Im not sure testing pressure at the gladhands would tell you much because it sounds more like a flow problem than a pressure problem... Although a simple way to check pressure would be to use a spare gladhand and find a air pressure gauge with male threads that can thread into the female side of the glad hand then connect glad hand to the red gladhand on the truck side. (Like ur hooking up a trailer or like ur hooking up a gladhand tire inflator hose)....

But if the brakes work fine as well then i wouldnt think it would be a tank problem either because all that air to the brakes is supplied from the trailer tank... There has to be a blockage on the supply line somewhere. It is either in the valves itself or in the line... And if u have replaced all the valves then it has to be the lines... I think if it is supplying air to the tank at a normal rate when u press the brakes and bleed it down, the problem is most likely after the tank itself.. Maybe a piece of rust inside the tank has made its way into the supply lines to airbag and leveling valve... Something as simple as unhooking the leveling valve airline and supplying air pressure to the lines (pressing in red knob, make sure wheels are chocked)may be enough to clear the blockage if their is one....

Just thinking out loud here. Hopefully this can help u a bit. Let us know what u discover

 

Didn't read all, but you did check the pressure protection valve? Should be in the left or right end of the tank.

 

Pass through valve? I had a similar issue. Could hear air hissing under hood sometimes, would hear it in the tube that runs under the cab. One time the brakes didn't wanna release on the trailer I'm assuming, but that was only once before I had the pass through valve on the firewall replaced. My shop I go to charged me I think $213 for the part, have to look to see what they charged for labor.

I could hear air through the valves on the dash though, for a short while after starting the day then it would go away. Now I only hear it long enough to charge trailer tank and no more air noise.

 

Curious to find if you ever figured out the problem

 

If u still have the old leveling valves , dont throw them away unless you end up stripping out the hex bolts that hold the cover togrther. You can rebuild them. If you open it up there is a couple of o rings inside that go bad then it leaks air.. Replace o rings then u have a new leveling valve or atleast one to keep as a spare... Theres a metal plate inside that is what opens and closes the different valves to control air... Make sure to put it back on the same way or it will reverse your air flow... Meaning the airbags will deflate when its supposed to go up and inflate when supposed to go down... Safe travels thanks for the update