Brakes won't release

I have a 1989 GMC 7000 Forestry truck and a couple of weeks ago the brakes started dragging just as I was leaving the dump. The pedal had no give to it. I bled the brakes to make it home and am now trying to fix it. It has disc brakes and a hydroboost unit. The electric motor is not coming on on the hydro boost unit. I am assuming that it is either the hydroboost or the master cylinder and I would like to change the right part the first time, as they are kind of high priced. Any ideas?