The fluid behind the pedal is because the cylinder is leaking, as I mentioned earlier. It's a common problem on old hydraulic systems and nothing unusual.
A secondhand master cylinder is, to put it politely, unwise. To put it less politely, it is downright foolish to fit equipment that not only may leak as badly as the original, but could in fact fail to work at all when you most need it in an emergency. You may be willing to risk your own life, but please consider the lives of other people in and around the car.