were the brakes recently changed on the car before you purchased it?
what happends on older vehicales (happened to mine)
is when you change the brakes the peddle goes down to the floor. the peddle fallowing the same range of motion, over time anything beyond that gets dried out. so your basically overextending the piston in the master cylinder and rupturing one of those o rings. happens, can be rebuilt. but buying a new one for 60 bucks, why go through the hassle.
Mc w/ ABS
good technique to put a block of wood behind the peddle so it doesn't hit the floor. but if you replace your master cylinder this shouldn't become a problem again for another 20 years.