Treadle Valve issue

Most likely bad foot brake valve.

 

You sure no air leaks.
Grab another person have them step on brake, while you go under truck and listen.
I had a similar problem.
Steer axle brake line was leaking.
The hole faced down .I couldn't hear the leak sitting in seat with the door open.
Good luck

 

If its not leaking out the treadle valve exhaust port, that's not your issue. I bet you've either got a hole in an air line, a leaking relay valve or a service diaphragm that's torn.

 

Could be a few things, even a dirty hose in the push fitting. You need another set of ears in a quiet area.

 

Sounds like the 2 way shuttle valve between the treadle valve and the trailer hand valve is bypassing. This valve's outlet is connected to the trailer service line and is supplied by both the hand valve and treadle valve. Its purpose is to utilize the highest air pressure to supply the trailer brakes. This is how the trailer hand valve is able to send higher brake pressure to the trailer independent of the tractor. Because air valves have the exhaust port open when not in use, it makes sense that if that 2 way shuttle valve is bypassing, it can send air back through the treadle valve and out its exhaust port when you pull on the hand valve.

 

Also I just re-read your post. The air leak you hear in the dash with your foot on the pedal is likely air bypassing that 2 way shuttle valve and dumping out the hand valve exhaust since that line usually gets routed back into the dash or just outside the firewall.

Great diagnostics in your post BTW. Makes it easier to come up with logical answers when we see a lot of the picture.

 

Treadle Treadle Treadle
I made it out of clay.
And when it’s dried and ready,
Oh Treadle we will play.
Treadle Treadle Treadle
I made it out of clay.
Repeat