1989 Buick Riviera Occasional soft pedal

Brakes problem
1989 Buick Riviera 6 cyl Front Wheel Drive Automatic 125k miles

Sometimes when I'm driving I go to slow down and the pedal goes almost all the way to the floor. It's not predictable, and probably not safe either. When I let up on the brake and apply it again, it's as firm as can be. I'm thinking brake booster, but some people tell me to do the master cylinder first. I'm wondering which I should really do first.

Also, could it have something to do with the multiple vacuum leaks in many places under the hood?

And finally, I'm thinking about adding a supercharger to the car. What else would need to be done besides strapping in the new charger?