I recently bought a 1985 GMC 2500 Sierra diesel. I has the hydroboost brake system but it isn't working. When i try to brake the pedal almost drops to the floor with no resistance, and then maybe the last 1/2 inch it applies a little bit of brakes but not enough to drive safely. I replaced the calipers and shoes thinking that might be the problem, I did bleed the after. then still thing. I just replaced the master cylinder, and while it was off I had someone push the brakes and I could see the part (No idea what it's called, but that pushes against the master cylinder) and it was working fine. After installing and bench bleeding the master cylinder I still drops to the floor with almost no resistance.
What else could be the issue>

 

Anyone have any thoughts?