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post #1 of 4 (permalink) Old Aug 9th, 2011, 12:51 pm Thread Starter
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Master Cyl Rebuild Necessary?

My rear master cylinder piston is not springing back after being pressed in. There was some corrosion residue around the rubber dust boot but there is no fluid leaking out. Would a rebuild be my best remedy? Thanks.

Last edited by jtimc7; Aug 9th, 2011 at 12:56 pm. Reason: To clarify
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post #2 of 4 (permalink) Old Aug 10th, 2011, 4:33 am
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I just replaced mine because I spotted the start of a fluid leak.

There are no oem parts available to service the rear master cylinder. My bike is a '97 and I've already made my mind up that clutch and both brake master cylinders will be replaced if they give trouble (because they have been very reliable). I'm ignoring boneyard replacements because they could be of similar age and wear. The master cylinders get a lot of use so when the rear started leaking, I decided to go with new oem.

I shall dismantle the used unit and might try to source a new seal just for a temporary spare. But if the new replacement lasts another 13 years, then either me or the bike may be long gone!

You could try removing the circlip and pull the insides apart, but being an important brake part perhaps a new oem is the best route. I think you will be paying around $140.But if the master cylinder is not returning and you burn out pads and a rear rotor, the cost will be much much more.

...... I've now partially dismantled my old rear brake master cylinder. Most of the sealing is done by two seals, the usual cup type pump seal and a second flat cross section circular seal in the master cylinder bore. The seal doing the work for braking is the cup seal.

The piston itself slides in a ptfe liner which whilst easy to press in, looks difficult to get out. The piston looks as though it's made of cast ali and could corrode if there was water trapped in the rubber boot. I've always filled my boot with grease. If yours is sticking it looks like it could be betwen the piston and this ptfe liner. You might get lucky removing it and cleaning out any gunk, but if the piston has signs of corrosion - forget it and buy new.



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Last edited by voxmagna; Aug 10th, 2011 at 8:26 am.
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post #3 of 4 (permalink) Old Aug 10th, 2011, 10:28 am
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Not too sure what kind of bike you are asking for, I'll assume it is the K75. BMW had two sizes of rear master cylinders over the years, a 12mm and a 13mm, depending on which one you have there may be some rebuilt kits available. Beemer Boneyard lists the 13 mm Rear Master Cylinder Rebuilt Kit for $50.00 or so.

http://www.beemerboneyard.com/34311454438.html

But that is only the piston/booth/spring, so if any damage to the cylinder you may be out of luck on that one. If it isn't lined there are some sneaky ways to polish/clean the bore but altough I was succesfull before doing so, I won't recommend them here.....just yet.

But all them motorcycle brake cylinders can be relined, there are a couple outfits doing so, however the shipping costs/downtime may swing you toward a new OEM, also available at Beemer Boneyard for a lot less than BMW, again depending on size/year, you'd have to doublecheck the correct fitment.

http://www.beemerboneyard.com/remacy.html

But....you could be lucky and it is only a broken/corroded/weak return spring in there. On a bike that old, I would be opening all the calipers and master cylinders.....but that's me and.....I have found corrosion before on bikes that were not even very old and had frequent fluid changes. I like to catch that before problems on the road or having to buy expensive metal parts vs the cheaper rubber bits.

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post #4 of 4 (permalink) Old Aug 12th, 2011, 11:19 pm Thread Starter
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Thanks gentlemen for your helpful insights....

(It's a '91 K75. It has 15,500 miles on it and it has been stored inside.)
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