I was backing up and noticed that my brakes were really spongy and I had to press the pedal all the way to the floor to get the car to stop. Then the brake light came on, went to get some brake fluid, thinking that was all it was. Filled it with brake fluid, brake light turned off. Started to drive home, noticed brakes still spongy feeling, but were bit better, got home after 5 minutes and brake light came on again. Opened hood and saw that brake fluid was empty. Guess is that it's a broken line, but am not sure. Also don't know what a master cylinder is, but was told that might be it. How much to replace a brake line...or for a master cylinder. How long do I have before it absolutely needs to be fixed.