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The abs has an inversion valve. Not sure if that would cause your symptoms though. If you had holes in your service brake diaphragms when you stepped on the brake it would cause a fast air loss.

Rears are spring brakes air keeps them off the treadle takes the air away when pressed. So they really can't just not work. Unless air is somehow leaking thru the treadle valve and the flow does not stop when you step on the pedal. Chances of mechanical brake failure of 4 separate spring brakes...

Unless your brakes are caged there should never be a situation where you step on the brakes and the truck does not want to stop. The treadle should be firm with or without air in the system. Spongy is bad.

Does the treadle valve feel spongy or different than normal. M939 treadle should be really firm.

Sticking unloader or leaky air dryer most likely

More information would be needed. Did you look for leaks? How slowly do you build pressure does it come up to 120psi? Was it working before or did you get it this way? Do you build enough pressure to release the brakes?