brake issue

1977 Chevy Silverado. Power brakes. Brake pedal goes all the way to the floor. No fluid noticeable around the master cylinder or vacuum booster. Turn the engine off, push the brake and it has some resistance, turn the engine on, and the resistance is gone and as the pedal goes down, you hear a pretty good "Whoosh" coming from the hole that the pedal lever goes into to activate the vacuum booster. I haven't looked at the individual wheels yet, there was very little master cylinder fluid level change before, during or since this has occured.

QUESTION: Should I suspect the master cylinder even though there is no fluid leaks, or the vacuum booster?

It is possible for the master cylinder to leak internally. The problem will cause no fluid pressure to the wheels, but no external leaks. Ray

i'll 2nd on it, got a 76 doing it now,, one test start it , hold brake down
let up quick (half way) hit pedal quit, don't let pedal stop, does it have
more pedal??? by doing the short stroke hit you put some pressure on
both sides of the piston cup's did it for yrs in VW's :lol2: :lol2: