Bruce B.

Is it POSSIBLE that air is leaking through the system from somewhere else?  For instance, if parking brake release air is leaking past the seal into the service brake chamber, it would exhaust out of the treadle while the pedal is released.

Does it leak when pressed, released, or all the time?

With parking brakes applied or released?

More info makes for easier troubleshooting!

I will check the parking release valve and the 2 other emergency valves. (red ,blue, yellow)on the dashboard. The treadle valve leaks all the time because when the truck is parked the parking brake air valve is pulled out. Today I checked all lines going to supply and all lines going to deliver. Will I have to plug each line in deliver 1 by 1 to check for back feeding?  

There are only a few ways air could get to the treadle.  First thing to do might be to block the main supply line coming to the treadle.  If that stops the leak, it probably is another bad treadle valve.  If there is still air coming out, you could then try to see where it is coming from.

You eliminated the parking brakes leaking through, so that's good.  In some systems, there is a line connecting the yellow valve to the delivery side of the foot valve.  This is an anti-compounding setup.  If air were to leak past the yellow valve, it could exhaust through the foot valve.

If changing the foot valve has corrected this issue in the past, there is a good possibility it will correct it again.  Then the question becomes why the valves keep dying on you.  One of the biggest culprits is oil in the air system.  That tends to kill the seals in the valves.