1997 Ford Thunderbird Brakes quit functioning

I was driving around town and my brakes were doing fine. I pulled into a fast food place to get breakfast and when I pulled forward the brakes went almost to the floor and the car continued to roll. As I pulled forward the same thing occurred. I drove to a very close store to get brake fluid. But it was ok. The longer I drove the car. The brakes functioned ok. But if I used them quite often they would go almost to the floor and would still coast. As I parked the car. I noticed that when I pumped the brake pedal. I could hear air whooshing when I hit the pedal.
What could be my problem?

thanks