I am working on something (not car related) and I need to find 3/4" cup seals like are used in master cylinders. I could get a rebuild kit if I knew an old car that had 3/4 inch seals. Mcmaster sells these but nothing below 1 1/2"
The NAPA website shows a lot of rear wheel cylinders, selectable by diameter (including 3/4"). They also appear to have rebuild kits. I didn't spend much time on the site to see how easily it is to connect the wheel cylinder to the appropriate rebuild kit containing the cups.