Ive stripped and rebuilt the master cylinder. There was not much to find, but the action does feel better now its all back together and bled yet again.
Its quite easy to do.
1 - Remove the mater cylinder from bike and remove the lever assembly.
2 - Unscrew the fluid cup - two torx head self tapping screws.
3 - Use an M6 socket cap head screw and small G clamp to push the piston down a few mm.
4 - Use a small screwdriver to carefully extract the dust cover. The sides are thin so carefully pick at the base. There is a little knotch just for that job and no circlip.
5 - The washer inside will now slip out.
6 - Release the clamp, extract the piston and clean everything.
7 - If the seals or cylinder bore are worn, it will need a new master cylinder. BMW do not sell replacement seals.
8 - Mine looked ok so I put it back together.
9 - Use the socket cap screw and G clamp to hold the piston down and refit the washer and dust seal. The seal is enough to keep it together, there is no circlip.
10 - Replace the fluid pot. It fits better one way around, so make sure its sitting flat. They are self tappers so turn the screws back until the threads click then screw up.
11 - Replace leve with a spot of red grease on the ball end.
12 - Refit mater cylinder to bike and bleed the system.
13 - I noticed tiny air bubbles would pop into the reservoir when the lever was moved slowly. Who knows if this was the problem all along
BTW, I never needed that 2nd bottle of hydraulic oil.
