Nick,
I've had three of these CNC master cylinders in my car for about ten years:
click here. They don't get much simpler than this. If the seal in your master cylinder is leaking, I'd look for signs the bore of the cylinder is becoming oblong rather than round. That can happen if the rod from the brake pedal to the master cylinder is not aligned properly, causing up/down or side forces each time the pedal is depressed.