Tim,
I'll be interested in seeing what people have to say.
Your symptoms are exactly what my 54/55 350 GMC COE did after I replaced all wheel cylinders and the master cylinder (and new brake shoes, adjusted properly). I bled the entire system 4 times (about 2 quarts of fluid) using a pressure-bleeder, all as described in the GMC maintenance manual.
I drove it about 15 miles to storage and I had to pump the pedal in anticipation of braking; the pedal got quite hard and braking was excellent. Nonetheless, after braking, the pedal went to the floor each time.
I had the hydrovac rebuilt but have not yet installed and tested it.
Tim
