under tank m.c. rebuild

[arbalest]

My son has a 76 R90/6 with the single disc ATE front brake. Since he got it, nice hard lever, but not stop. Just to recap, bike has 21,000 original miles. Suspect original brake pads and fluid. Replaced the pads and flushed the system with new DOT4 brake fluid. WOW. It now had a front brake. The problem is, that over the next two weeks, all the fluid leaked out of the m.c. Very slow leak. Loses all the fluid in about a week. Why did it hold all the original fluid just fine, until we flushed it out with good fluid? I have the BMW rebuild kit. Will let you know how that works out.

[ME 109]

Inspect the bore of the master cylinder thoroughly for pitting and other corrosion related damage.
No point putting a new piston and rubbers (kit) in there if it will be damaged, and return a much reduced service life.

New master cylinders are reasonably priced at Motobins.......

[Seth]

I had the same one one of mine. The master was a single disk (13mm) that I had previously had machined with a second port to use on a dual disk setup, so I wanted to save the master. I bought a hone like this:
http://www.ebay.com/itm/350557445958
and it cleaned it up enough that I have no leaks after installing a new piston and seal kit.
But yes, if the damage is too bad, you'll need a new master.