K75 master cylinder

[Lbxpdx]

I just picked up a parts bike, a 1994 K75S ABS, but I think it might turn out to be a minor project bike.  One thing I noticed was it was leaking fluid from below the lever of the master cylinder.  I have zero history with disc brakes.   It would appear from the disassembly that the o-ring was compromised with time, causing it to leak down the screw hole that affixes the reservoir to the housing.  I’m wondering if my cups look okay, or do I need to order a whole rebuild kit?  There is no putting on the cylinder walls. 
Any and all advice is welcome.   

[Lbxpdx]

Well, I did some cleaning up and I’m going to get a new o-ring for the reservoir and see how it goes.

[DC950]

How did it go?  I've never replaced just the o-ring on other bikes (KTM/Magura) but had to replace the complete piston assembly.

[Lbxpdx]

I forgot to order it.  I’m placing order the order with BMW tomorrow.

[matjas]

had a similar case in r1100GS - these cylinders when they go are hard to get working again - you can clean this and hone the cylinder slightly with scotch pad on a stick and a drill - goes very nicely.
ONE THING to remember though - the bore has been anodised - what youre getting now once the anodising is gone, is soft metal so the longevity might not be great. I still need to see how it goes in my case.

BUT - If the cylinder keeps leaking I am going to bore it out to oversize /there is plenty od meat around/ and sleeve it with a stainless thin walled sleeve loctited inside the bore - this will be a forever solution.

best regards
M