Had the same issue with mine. The prop valve had stuck in the safety position. I took my valve apart, blew air through it, tried every thing I could think of... NO LUCK... ended up going to the local salvage yard and grabbing three prop valves.

I bench tested one with compressed air and it blew clear all ports. Installed in truck and ten minutes later I had all the brakes bled!

Took a few weeks to straighten out, but things look good now thanks to the folks on here!