brake problem on 77 Silverado

Pedal goes to the floor, no fluid leakage at master cylinder or vacuum booster, no brake fluid level problems. When you push the brake with the engine off, you get some resistance, turn the engine on and the pedal sinks to the floor.
Question? Normally a master cylinder will leak when it goes out....but is this my prime suspect or should I suspect the vacuum booster?

master cyl. if the booster goes it will leak vac. you should hear it,and/or
engine run like one of the big vac.line is off :smokin:

Did you try bleeding the brakes? If you did, and it's still the same, it could be that the master cylinder is worn out (possibly pitted) on the inside, or the o-rings aren't sealing. If that's the original master cylinder, that's probably your problem. If your vacuum booster goes out, when you take it for a drive and try to stop, you'll have to apply two-leg pressure on the pedal to stop the car. My booster went out while I was driving, and WOW...how exciting it was to stop :). It's WAAAAY harder than the old non-power brake cars. Don't even bother trying to rebuild your master cylinder. I did that on a couple of cars, only to end up replacing it anyway. New Master Cylinders are fairly cheap.