A few seconds after brakes are applied the pedal starts to fall to the floor. When I pump the brake pedal I here a hissing noise coming from the vacuum booster.

Like emissionwiz said you have both out but replace the master cylinder first, i said replace the Booster check valve not the booster, the check valve can do the same thing, you can also buy a used one from a junk yard and replace the booster that way. but check the check valve first. it is inlineon the hose going into the booster. Start engine and run pull booster hose out and hold end of hose turn engine off does the hose still have vacuum? if yes then it's booster valve if no then it's booster diaphram. Like emissionwiz said you have both out but replace the master cylinder first, i said replace the Booster check valve not the booster, the check valve can do the same thing, you can also buy a used one from a junk yard and replace the booster that way. but check the check valve first. it is inlineon the hose going into the booster. Start engine and run pull booster hose out and hold end of hose turn engine off does the hose still have vacuum? if yes then it's booster valve if no then it's booster diaphram.