Sounds like your master went south. Try bleeding the system starting at the master, then the valve, then the wheels farthest from the master. If it bleeds out, you may get air again if it sits for long periods of time. This happened to me with a new master. I had to bleed every spring for a few years until the master or maybe the wheel cylinders "broke-in". I have'nt had a problem now for several years (except with the Corvette).
Good luck pinning it down,
Scott
