15mm front brake master cylinder

[John Roberts]

This is on my '81 R100 RS. I have a very small but persistent leak from the O ring seal between the reservoir and the master cylinder body. This is due to corrosion in the socket in the alloy body of the master cylinder into which the stub and the O ring fit. It's purely a cosmetic issue, there's (virtually) no pressure behind it and it isn't part of the high pressure braking system, it's just on the reservoir side. In fact I haven't ever noticed a drop in the level of fluid. A new replacement item is only available complete with the reservoir and the piston for £165.

The amount of corrosion is very small but enough to allow the seepage, does anyone have any suggestions to stop the leak- is there a sealing compound available to back up the O ring?

 

[Edward]

This is on my '81 R100 RS. I have a very small but persistent leak from the O ring seal between the reservoir and the master cylinder body. This is due to corrosion in the socket in the alloy body of the master cylinder into which the stub and the O ring fit. It's purely a cosmetic issue, there's (virtually) no pressure behind it and it isn't part of the high pressure braking system, it's just on the reservoir side. In fact I haven't ever noticed a drop in the level of fluid. A new replacement item is only available complete with the reservoir and the piston for £165.

The amount of corrosion is very small but enough to allow the seepage, does anyone have any suggestions to stop the leak- is there a sealing compound available to back up the O ring?

John, I had a similar problem with my 83CS and the only way I found to resolve it was to buy a new one a couple of weeks ago. For what it's worth Motobins were about £20 cheaper than Motorworks and supplied a BMW original part.

 

[John Roberts]

Yes, the depth of the corrosion is so small it's annoying, Motor Works want £177 and Moto-bins ask £156 which is fair enough I suppose, but I only need the cylinder part, not the whole thing. I feel you pain but I'll see how mine behaves. This does seem to be a pattern with Airheads (well, mine anyway); it goes along happily for yonks with only the annual service then out of the blue, Wham! I hope that disturbing the joint and cleaning the corrosion off hasn't made things worse. I'll let you know how it goes for the general good, feedback often makes a thread a lot more helpful.

I was offered a used assembly by MW or MB, but I thought the chances would be that that would also have at least the beginnings of corrosion in the same spot.

 

[Dennis the Menace]

You could try Loctite, they do a sealant which is as thin as piss ( i think it is green in colour ) clean the parts, assemble,then use the Loctite, it then seeps into the joint, it is brake able after it has set, if and when needed
Phone Loctite technical, they will point you in the right direction

 

[John Roberts]

You could try Loctite, they do a sealant which is as thin as piss ( i think it is green in colour ) clean the parts, assemble,then use the Loctite, it then seeps into the joint, it is brake able after it has set, if and when needed
Phone Loctite technical, they will point you in the right direction

I like that idea. I'll call them tomorrow.

 

[chasbmw]

If you do replace it, I think that it would be worthwhile to move down to a 13 or 14mm cylinder as fitted to the Brembos on the last airheads. Might need to move to a twin pull throttle.


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[BillN]

that reminds me - I need to look under the tank of my R90S

I though that BMW did not stock the brake master cylinder anymore .......... i.e. gone for ever