MZ/MuZ Riders

I have a 1993 ETZ 251 which I have owned for about 10 years, the bike has only covered about 15000 miles. The brake master cylinder continously leaks (weeps) at the seal to the piston rod, I have fitted numerous kits of seals, new and pistons. The brake its self works very well with no loss of pressure and the cylinder casting is in good condition with no evidence of wear. The outer seals, even when new are not particularly a tight fit on the piston rods !

Is there a fix for the master cylinder as I understand that new replacements of the MZ item are not available ?

I suspect you have two options, a second hand replacement, or replace with a master cylinder from a jap bike. The important thing to remember if going the jap route is to match the piston size. On the plus side of course is that jap levers are more easily available.
P.S. just found this in germany: http://www.ost2rad.com/MZ-Spare-Parts/Driving-mechanism-Brakes/Master-cylinder-for-brake--MZ--ETZ-125--150--250--251--301.html
P.P.S. Are you using the correct type of brake fluid for the system, some of the silicon based ones will attack the rubber seals.

Been using mineral based brake fluid so should be OK and have never had any leaks from the seals in the caliper.

I wanted to keep the bike to original design if possible.

In the past I had a suggestion to fit a washer between the outer rubber seal and its retaining circlip, to help support the seal, but have not tried this as I assume that a very thin washer is needed. Has any one used method this with any success ?

The Grimeca brake is used on the Harley Davidson Rotax 350 and probably many other (mostly Italian-made) bikes. Don't let the name on the wheel cylinder put you off, if you gave Grimeca a big order you could have them with your name on.
You seem to have tried the obvious things. My 70,000 mile version has never leaked a drop. Which does remind me that I really ought to change the original brake fluid.
The site of the hole in a porous casting can be very hard to find. Norton Commando owners were advised to clean their cylinder heads thoroughly and dust them with talcum powder.
Good luck
Louis