Most likely the master cylinder, but check the brake booster too. (That big black disc-looking thing with a vacuum hose coming out of it behind the MC) With the engine off, pump the pedal repeatedly. Each time it should get firmer and firmer. If not, the booster is bad - Replace it and the MC. If it firms up, press the pedal down halfway and start the engine. If the pedal goes to the floor, then the MC is probably the culprit. I'm not sure, but I believe moisture in the hydraulic lines can make it mushy too, since it's "Squishy" like air, compared to hydraulic fluid, which is very dense. Easiest way to get rid of it is to flush a gallon of DOT3 from a sealed bottle through the system using standard bleeding techniques.
Just my $.02, hope it helps!
Peace,
Matt {peace}
Just my $.02, hope it helps!
Peace,
Matt {peace}
