My first action would be to change the fluid. My brake pedal would go to the floor board and hardly stop when I got air into the lines. One time I only had rear brakes but that was from boiling the fluid repeatedly in the front calipers. Felt/is the same as air in the lines, pedal to the floor and almost no brakes at all. I've never had a master cylinder fail and don't know the symptoms but it sounds plausible too.

Les