'hate to tell ya this, but it ain't yer vacuum booster. All that does is amplify the pressure to your brake master cylinder, and if it failed, you should still be able to lock up your brakes by mashing really hard.

http://auto.howstuffworks.com/power-brake2.htm

If the vacuum booster failed, the pedal would just get really stiff. It won't fall to the floor unless either the fluid leaks somewhere, or if air is coming back into the lines somehow and compressing when you mash. It has to be leaking somewhere. Mark your brake fluid level, and after your car does this crazy shit :loco:, park it back where it was when you marked it (to ensure your car is at the same level) and see if the fluid level has dropped any. I bet you'll be surprised at what you find. Perhaps the adjustment of the brake master cylinder rod is not right?