I don't know all that much about the various fluid types, but...I used to have similar problems which were almost always improper rebuilds on my part. That aluminum just doesn't seem to hold up so well against scratches and corrosion.

Solution...stop rebuilding and buy a new one (or a professionally rebuilt unit). I did that in my 1800s more than 200,000 miles ago, and it only recently started leaking. Just a bit though. I have to top it off perhaps every 10,000 miles. I figure I would have had to rebuild it at least 5 times in that mileage... That would be about $50 worth of kits (if I could find 'em) plus brake fluid plus the time and gross brake fluid all over.... And the new one cost me somewheres around $30. That was, of course, 7 years ago...but I think it is a fairly common part on lots of European cars (mostly British), so it shouldn't be too hard to find....

-Matt