If the pedal can go to the floor then it's not a booster problem.

Does the brake pedal go to the floor when the engine is off?

Most likely the master cylinder, check for any external leaks at rubber hoses and calipers as well.

__________________
Alex.

MB Tech
Sydney, Australia
Volvo 122S
W201 190D 2.5 manual
W202 C240
W203 C32