I need a small master cylinder with 7/8" bore

Doosterfy,
I agree w/ your analysis. I also doubt the MC has anything to do with the problem. Even if it were originally used in an X-split car (not K-cars), that would only affect the car's plumbing, not the MC itself (save for reservoir sizes which only matters over the long term). Glad you apologized for your frustration and are learning to filter and weigh each response. Many OP's here never man-up. Most replies on this site are spot-on, but some are just vague speculation, though all can be useful in solving a problem. I can't even count the number of times I have said "can't be that", but turned out to be. My wife (knows little of cars) once mentioned I should try new spark plugs when I had a mis-fire. After discounting that and swapping many other ignition parts that turned out to be the fix (sort of, needed smaller gap too).