The best approach to repairing this problem is to simply replace the master
cylinder assembly with a new old stock unit. In the US these are inexpensively
available from a variety of sources. When I helped work on my neighbors
GPW jeep (same car) he was able to get fuel pumps, brake cylinders,
master cylinders, transmission parts, all new old stock. So one thing you might
do is search around and try to get plugged into a network of folks who work
on military vehicles like that and they might be able to steer you to a source.
Or, do a bit of legwork and find a supplier on your own.
It's nice to have new stuff on the vehicle as it makes it a much more reliable
driver in the long haul.
Now.
If you cannot find a replacement unit, then inspect the rebuild parts you will
be putting into it if you do rebuild it. The bore is probably a round number in
sixteenths of an inch and if so then you can purchase brass telescoping
tubing from a company called Small Parts in the US. This is how I did it when
I had to sleeve a cast iron master for a motorbike which was tough to get.
The bore of the tubing will be near net size for your piston and cups if you
are lucky.
So the drill is, bore out the ID of the cylinder, ream to size with a one over
reamer that allows a thou or so on the radius clearance for the tubing you
decide. Cut the tubing to length, deburr the ends. Degrease it all very
well and coat the exterior of the brass with locktite 600 series bearing
retention compund. In my case I cut some *very* shallow grooves on the
OD of the tubing (two or three thou only) to aid in retention.
Then drop it in, let it set up.
Drill the ports (can be tricky, some of them are very small and at an angle) and
then deburr the holes into the bore that you have made. Otherwise the
rubber seals will get torn up.
Jim