Just completed install of new calipers, pads, rotors, front wheel bearings on a 95 CTD 4x4 club cab 2500. While bleeding brakes, just when it appears almost all air is out of the lines, more air appears ( as if one was just starting the bleed process ). I was pumping the pedal to bleed with the engine not running. Started engine ( bleeders closed ) and pushed on brake pedal. Pedal which was quite firm and quite "high" went almost to the floor. Pumped up the brakes and kept pressure on the pedal, and it slowly squeezed down til the pedal was almost to the floor. Tried this several times, one after the other, same result, after a few seconds, pedal goes almost to the floor. I have put almost 2 litres ( quarts ) of fluid through the lines, there cannot be that much air in them. Last time I replace a rear brake line, I never had this difficulty. Could this be a master cylinder problem? There does not seem to be fluid leaking anywhere. Fluid level does not go down in the resevoir when this occurs. I was driving the truck ~20 miles home from work when the rt front caliper started leaking. Sometimes when I stepped on the brakes there was absolutely no resistance ( along with no brakes ), other times there was almost normal resistance ( and normal braking ). As the caliper was leaking, I expected this was the problem, but would have thought it would have the same resistance all the time. Perhaps this was the master cylinder starting to go? Any thoughts? I don't really want to just replace parts if it may be the vacuum pump or the booster which is the culprit. Thanks, Rick

 

anytime your Master cyl, sinks to the floor, and there are NO external leaks, its the mastercyl thats at fault.
likely during bleeding you pushed the MC piston further than it has travelled and it hit rust, damaging the seal.

the brake booster, only aids your foot in pushing on the brakes, it would not have any bearing on allowing the brake pedal to go to the floor. (is yours a vacuum booster or Hydro boost?)

 

He has vacuum assist on his '95.
"Normal" problem with vacuum assist is the vacuum pump can't keep up with repeated braking at idle and will cause what you may perceive as a problem.