It was (or still is) a leaky clutch master cylinder. Would not always
leak, but by driving it more often, I noticed brake fluid level
slowly sinking and wet traces around bottom of steering column. The
leak is from top of clutch master cylinder, where the pedal pin goes
into. Did however also replace master brake cylinder and this time it
came with a proper bleeding kit and instructions. It is important to
release the pin slowly when brake bleeding, otherwise air will get
sucked into it, even with bleeding kit and hoses attached, and wait
about 10 secs between pushing the pin again. Took about 10 minutes
clamped in a vise, and MC has to be level, which means reservoir
won't be level and has to be topped off regularly during the process.

Thanks for everyone's help

Cheers

Matthias