Jessica, have you ever replaced the fluid? How is the cap on the master cylinder? Brake fluid is hughly hydrophylic, meaning it loves to absorb water. (This is why you always here to use NEW brake fluid when doing a brake job.) When water gets in there it can cause problems and mess with the pedal effort. Typically if the pedal goes to the floor there is air in the lines somewhere. I'd bleed them first and check the pad thickness. If those check out OK, then look for a leaking brake line. If that fails they make a pressure tester for brakes that you can hook up to each caliper and test to see how much pressure it is seeing. I've never used one of these, nor do I know where to get one, but I'm sure the local auto parts store can lead you in the right direction. Let me know what you find out after bleeding them and checking the pads. You do have enough fluid in the master cylinder right?
