brake line problem for 2001 accord

I changed the rear brake pads and I did the necessary things to bleed any air in the lines....then several days later my wife was driving and the brake pedal went to the floor, but the car still would come to a stop. After inspecting the car I found that the right rear brake was not braking. I checked to see if it was getting fluid and it was not. I am pretty sure but not totally that air in the line is not a problem and no obstruction in the line. I was wondering if the box where the main lines from the resevoir run to and then out to the brakes might have a problem. Anyway I just thought it was weird that the brakes worked fine for several days and then this happened. I do appreciate any ideas or help. Thanks.