brake system,Why is my brake pedal going all the way down to the floor?

When I start braking everything seems normal, but the more pressure I put on the pedal the farther down the pedal goes, at one point it will just drop to the floor. My brake light came on, and I checked the fluid and I was low. So I added more brake fluid, the light went off but the pedal continued to hit the floor. Why is it doing that? I know generally that brake pedals shouldn't hit the floor, and it seems when I pump the brakes it's ok for a few seconds, then the pedal just drops.

My car has 101900 miles.
My car has an automatic transmission.