JD404.., I think I had that same failure once. Thankfully I took notes. I think you have an o-ring leaking in the venting portion of the treadle valve. It is not a total failure situation if so. The leak will drain the air over time but does not involve braking so you can 'live with it' until you have no fires heading toward you. The o-ring is number 15 on the exploded view of the treadle valve (my notes). It can be changed without taking apart the treadle valve. There is a large C-ring that is accessible to remove to get to the o-ring. It holds back a spring so care is needed. It's 2:30 AM here so I can't run out to take a picture!
My notes reminded me I did not replace that o-ring, I flipped it over to move the flat side to a non-critical position, and the leak stopped. Of course I meant to find a kit and rebuild the treadle valve, but didn't, and forgot. I am old too. That fix was done a couple of years ago so lasted that long.
My notes included a test for that o-ring leak: On the treadle valve you see the four tubes in your photo above, 1/2 inch black nylon (whatever) tubes. The upper two are the air inputs from the two air tanks (not wet tank). The lower two tubes are outputs to the brakes. Remove the two lower tubes. You will probably hear air coming out one or the other. Put your thumb over the air source. If the leak moves to the other port then you duplicated the test I did. That shows the leak is common to both circuits (their vents) and that's why your tanks both drain down.
