Comeaud,
You are beyond me at this point. I can think of only two suggestions, both of which you probably have already looked into. First, are you certain that all the air is out of the master cylinder line? This was easy in my case because there is a bleeding fitting on the master cylinder to let air out, and it is all downhill to the booster slave cylinder. Second, if you have vacuum operated boosters, is the vacuum line dis-connected, plugged, or leaking? My truck came with a big hole burned in the vacuum line hose where it had been touching the exhaust manifold (this was the worst of several vacuum leaks). I agree with starting at the master cylinder so that you know what's new from the start. If weather permits, I will be removing my hydrovac unit this week for rebuilding before fixing wheel cylinders - since I only have one, it has to keep working so the truck will stop!
