Thanks again for the welcome every one
Berni - Yes the Tatra is very funky and you don't see many about
Neil - You might struggle to get any seals as most outlets disposed of seal kits for mineral oil some time ago, technically they shouldn't sell them for road going vehicles as they are not DOT approved for braking systems. If you do find some they will be NOS. If you decide to go for another NOS master cylinder make sure you strip it down, clean and re-grease everything before fitting otherwise you will take the edge off the seals on the first couple of strokes. In fact anything you use that is NOS should be treated the same way as 20 years on the shelf doesn't do them any good. I think I mentioned above that I helped out a friend over Xmas to set his engine up, he had got a NOS distributer still in the box never fitted, but it still took over an hour to dress the points and get the auto advance mechanism working as it should, and thats before the wonderfull task of getting the two sets of points in sync. with each other.
You are faced with two other options, either change both the master cylinder and slave cylinder to conventional seals and use normal brake fluid or just change the master cylinder seals and use synthetic brake fluid as the synthetic fluid is compatable with both types of seals, just make sure you flush out all traces of mineral oil.
Hope your back gets better soon as they are not the easiest of vehicles to work on, I've put on about 4 stone since I have done any serious work on the Stalwart and it certanally makes work hard going - time I went on a diet me thinks