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Thread: 83 K75S - Rear Brake Master Cylinder

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    83 K75S - Rear Brake Master Cylinder

    During
    Last edited by ghyber; 04-06-2014 at 02:49 PM.
    1993 K75S Mystic Red Pearl
    2007 K1200GT 997 Blue

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    Quote Originally Posted by ghyber View Post

    Is it normal to have some hydraulic fluid in the boot area

    or is that evidence of a small leak from an impending failure of the o-ring on the piston of the master cylinder?
    No. The boot is to keep dirt out, not fluid in.

    Yes. Replace the master cylinder, unless you know how to rebuild it.




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    Lee Fulton Forum Moderator
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    Registered User dieselyoda's Avatar
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    Yep, replace/rebuild the M/C.

    If you ignore it, you risk loosing residual pressure which means you have to pump the pedal to get firmness. At worst, she just goes and you loose the rear brake.
    1997 R1100RT (Restored Basket Case) , 1981 KZ 440 LTD (Restored Basket Case)
    1986 K75S(the beutch), 1993 K1100RS (blown engine), 1997 Chev Short Box (4x4 with an LT1)
    "You can easily judge the character of a man by how he treats those who can do nothing for him."

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    Instigator cele0001's Avatar
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    I have a similar situation on mine. I took it apart ,cleaned it, rubber looks good. We'll see. Haven't had a chance to test it long term, so far it is ok. It is worth a try
    I want to be unique, just like everybody else.

  5. #5
    Rally Rat
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    My rear master cylinder blew up a couple of years ago while I was riding. The brake fluid spray got all over the rear shock piston and destroyed the shock from the inside. Had to replace both the MC and the shock.

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    Rear Master Cylinder

    Rear master cylinder on my 90 K75 has been leaking a small amount for the last couple of years. I add a little Dot 4 couple times a year. Bought the rebuild kit last year but have been putting off doing it. The K75 has been moved to 2 Nd. place after buying a used 08 F800ST 2 years ago. K75 has about 153,000 miles on it but only got about 2,000 miles last year. Trying to decide if it's worth doing the rebuild or if when i tear it down I
    will find the bore all pitted and i will have wasted my time & the $75 price of the Kit. Has any one had any
    luck rebuilding one??

    Bill

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    Instigator cele0001's Avatar
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    Quote Originally Posted by tvbilld View Post
    Rear master cylinder on my 90 K75 has been leaking a small amount for the last couple of years. I add a little Dot 4 couple times a year. Bought the rebuild kit last year but have been putting off doing it. The K75 has been moved to 2 Nd. place after buying a used 08 F800ST 2 years ago. K75 has about 153,000 miles on it but only got about 2,000 miles last year. Trying to decide if it's worth doing the rebuild or if when i tear it down I
    will find the bore all pitted and i will have wasted my time & the $75 price of the Kit. Has any one had any
    luck rebuilding one??

    Bill
    Given how easy it is to open the cylinder it might have been a good idea to look at it before buying the kit.
    I want to be unique, just like everybody else.

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    Quote Originally Posted by cele0001 View Post
    Given how easy it is to open the cylinder it might have been a good idea to look at it before buying the kit.
    Hind sight is always 20 20 !!But I usually want to have parts in hand before starting a job. I got motivated enough last Friday that I pulled it apart & Installed the kit. Cylinder had a few shallow pits in it. Will have to see how long it last. Did about a 40 mile test ride. So far no leaks yet. Job only took about 3 Beers to complete.

    Checked on Beemerboneyard web site & they show a new Master cylinder for about $130 some dollars. Showed used ones out of stock.
    Bill
    Last edited by tvbilld; 03-31-2014 at 04:26 AM. Reason: forgot

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    Instigator cele0001's Avatar
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    It is amazing how much you can charge for a couple of pieces of rubber isn't it?
    I want to be unique, just like everybody else.

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    Hello
    Last edited by ghyber; 04-06-2014 at 02:49 PM.
    1993 K75S Mystic Red Pearl
    2007 K1200GT 997 Blue

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    Care to share the "logic" behind the "sitting for a few months will cause brake systems to leak" theory?



    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

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

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    Last edited by ghyber; 04-06-2014 at 02:50 PM.
    1993 K75S Mystic Red Pearl
    2007 K1200GT 997 Blue

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    So what is the acceptable amount of leakage on YOUR BIKE?

    How does that compare with BMW's recommendation?

    What would be the result of a failure on that particular system?

    A leak is a warning. Will you heed it?

    New brake seals in undamaged bores do not leak, even in cold weather. Worn or damaged seals or bores do.



    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

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

  14. #14
    Registered User dieselyoda's Avatar
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    That's actaully a very good, dead on answer.

    Most hydraulic systems will leak in the absence of residual pressure. It's usually due to lack of pressure to keep the cup expanded. Shrinkage of the seal, not so much but rather, any seal, requires a fluid film to be effective.

    Brake systems though, I tend to be over cautious and that is just my advice but if if you are comfortable and the machine stops when you need, perhaps I'm being a bit of a Granny.
    1997 R1100RT (Restored Basket Case) , 1981 KZ 440 LTD (Restored Basket Case)
    1986 K75S(the beutch), 1993 K1100RS (blown engine), 1997 Chev Short Box (4x4 with an LT1)
    "You can easily judge the character of a man by how he treats those who can do nothing for him."

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    Last edited by ghyber; 04-06-2014 at 02:50 PM.
    1993 K75S Mystic Red Pearl
    2007 K1200GT 997 Blue

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