brake problem

[wingzero]

about a week ago, I'm not sure exactly what happened, but I lost almost all the pressure pedal I had in my brakes, now if I slam the the brakes, it just feels really soft, the pedal goes all the way down, and it almost doesn't brake at all. I checked the lines and pads and they seem to fine, could there be something wrong with the master cylinder? any help would be greatly appreciated.