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Discussion Starter #1
Hello, first time posting here. I am the proud owner of a 93 K75 with 45k miles. had it since April last year and have been doing some well needed maintenance on it.

The previous owner has a great record of whats been done and when but I noticed brakes were just never on his list, neither was an air filter.

well, I decided to start with the "easy" stuff and change oil and flush brake fluid (till I can get new lines in and change the system all together). Oil was/is a mess including hot glue to act as engine gaskets (I am now rebuilding the engine to replace all that I can, thankfully no damage seems to have been done) but the main issue is the brakes.

The front was DISGUSTING but the rear was... cloudy. this seemed odd so I check the lines, it seems that my rear MC piston is STUCK. I mean its not going anywhere, I thought that it was a strange system I hadn't seen before because the MC seemed solid thats how stuck it is. I really would like to not replace the MC (as that is kinda expensive) and I would rather just repair, but if I gotta I gotta.

What I'm here for: anyone know a trick to get the MC piston unstuck? Or an affordable MC to replace with?

What I have tried: new fluid, compressed air, soaking it in vinegar, soaking in WD40, taking a torch to it. next up will be freezing it.
 

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FWIW: I've not seen much involvement with K75s here but if you go to Motobrick.com you should find plenty of folks--relatively speaking--who might address your problem....

Larry Johnson
 

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If you actually do get it apart, I think you'll find that the bore in the cylinder is trashed. I had a problem with my original rear cylinder on my '85. A new master cylinder cost double what the rebuild kit cost, but it was worth it since the original was pitted and would not have resealed with just a kit.

Frank
 

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Discussion Starter #5
Duckbubbles, I had the feeling that would be the case but was hoping someone would say "this is how I did it and I saved money!" Thanks though!
 

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I concur about the cylinder being pitted if the piston is seized. I have installed a rebuild kit, only to wind up replacing the master cylinder because even after honing the new seals didn't seal in the damaged cylinder. Recently I had a front master cylinder from a 1974 R90/6 bored out and re-sleeved at Apple Hydraulics in New York (Apple Hydraulics). It was only $95 for the work, then I installed the BMW repair kit (new seals and piston). The brakes work great now. It might be worth checking with them. They specifically identified the old airhead MC's for re-sleeving. I'm not sure if they will do it to any MC.
 
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