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Thread: 1987 K100RS- Rear Master Cylinder Rebuild Kit

  1. #1
    On the Road
    Join Date
    Dec 2012
    Akron, OH

    1987 K100RS- Rear Master Cylinder Rebuild Kit

    My rear master cylinder is losing a few drops of brake fluid each day, and my rear brake could definitely be more powerful. I was curious to know if anyone had used the rebuild kit that is available for these bikes. If so, how were the results?

    Additionally, I see that there are 2 kits available: 12mm Magura and 13mm Magura. Can someone tell me which one would be the correct fitment for my 1987 K100RS?


  2. #2
    Registered User
    Join Date
    Mar 2008
    Belton Tx
    Look on the body of the cylinder for the size. You may have to remove the cylinder to see the marking.

    91 K75RT ABS

  3. #3
    A bozo on the bus deilenberger's Avatar
    Join Date
    Mar 2003
    Spring Lake NJ, USA
    IIRC - the size is marked in a circle. You probably have the 12mm if your bike has ABS. Using with your VIN# should give you the correct part number.

    As far as your other question.. it depends.

    IF the brake fluid has been changed on schedule (every 24 months from date the bike was manufactured) it's possible your cylinder is still in good shape. Cleaning it up with some 1,000 grit paper (with brake fluid on the paper), then cleaning it up carefully might make it a good candidate for rebuilding.

    IF the fluid hasn't been changed regularly - the cylinder is the lowest point in the rear brake system, and moisture will accumulate in the cylinder causing corrosion. Fixing that is beyond the scope of a cleanup. The corrosion usually leaves pits in the cylinder which will quickly destroy a new seal resulting in the same (or worse) leaking you now have. On a bike that the cylinders aren't available anymore, there are speciality shops (catering mostly to classic cars) that will bore the cylinder and fit a brass lining to it, but that's not cheap, and would likely cost more (with the rebuild kit) than a brand new cylinder.
    Don Eilenberger
    Spring Lk Heights NJ NJ Shore BMW Riders
    '12 R1200R - I love this bike!

  4. #4
    Mars needs women! 35634's Avatar
    Join Date
    Apr 2008
    SW Ohio
    Yours should be a 13mm, they didn't have ABS in 87, did they? this is from
    Attached Images Attached Images
    1987 K75S
    Original litter
    Original owner
    2012 Ural Gear Up

  5. #5
    Registered User robsara's Avatar
    Join Date
    Dec 2009
    West Michigan
    FWIW, after I bought my K75, before I had my cycle endorsement, my rear MC was leaking. I asked about it where I bought my bike. The BMW dealer said they would fix it. They said there was a rebuild kit available, but didn't recommend it. They instead recomended a new MC.

    Since it was their dime, I let them fix it their way.

    I'm sure you will have other opinions, but I thought I'd take the time.
    Rob R
    1994 K75 My first bike.

  6. #6
    Registered User beemerguru's Avatar
    Join Date
    Mar 2003
    Foster City, CA
    On the bottom of the MC is the number 12 or 13. Still available from BMW.

    You have a 50/50 chance of success on a rebuild. Easier and safer to get a new one. Might check the hoses for age issues as well.
    Greg Hutchinson
    R80G/S (4) 633CSi with 450K mile
    '68 R60/2 '88 K100RS Special Edition

  7. #7
    3 Red Bricks
    Join Date
    Jan 2007
    Pleasanton, Ca.
    Beemerboneyard has complete new master cylinders for $129.

    On K100s, 13mm was used to 10/88. 12mm after 12/88.


    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

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