Also for the master brake cylinder a revision kit was obtained (via www.ccparts.nl) for only € 31,-
Last but not least 2 Liter of new brake fluid (DOT 3 & 4) was used. And presto, all working excellent again.
As for ATE, http://www.ate-brakes.com, I learned they have a very good online service for their classic parts. Might come in handy.