I pressed the brake pedal there were no brakes?

I was driving my 1999 Deville and went to stop and I had no brakes. Fortunately this happened in my apartment parking lot and nobody was in front of me! When I pressed the brake pedal there were no brakes and the pedal just kept going down to the floor. When I stopped I noticed a brake light on the dash and a message that said check brake fluid. The fluid was way down so I bought a 12 oz bottle and it took it all and was just above the full line. However, I still had no brakes. I think this happened to me once before some years back in another cadillac that I owned and it was the master cylinder. What do you think may be wrong? If it is the master cylinder, can you give me an estimate as to what it will cost to fix? My car is nearing the end of it's life and don't want to spend to much on it. Thank for your help.