Brake pedal going to the floor is most likely a fluid leak, either interal in the master cylnder or external in a leaking metal brake line, brake hose or caliper. In some cases, if a seized caliper gets hot enough, the brake fluid can boil and may cause a low pedal.

Usually if the brake booster fails, it will cause a very hard brake pedal and require heavy pressure to stop, sometimes accompanied by a rough engine idle due to a major vacuum leak at the failed booster. I wouldn't worry until it fails.

If they haven't been done in the last 10 years or so, I recommend all new brake hoses. They can look OK outside but be swollen shut inside, holding fluid pressure and causing the same effect as a seized caliper. Recheck the calipers after hose replacement. If any are still seized, rebuild or replace them in pairs, to maintain equal braking on both sides of the car.

Happy Motoring, Mark

__________________
DrDKW