Rear master cylinder size

Discussion in 'Airheads' started by Eduardocastrejon, Mar 16, 2020.

  1. Eduardocastrejon

    Eduardocastrejon Been here awhile

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    Hi
    Please help me with this
    Some months ago I rebuilt the rear master cylinder on my 81 r100rs, like always was hard to purge but is braking, i tried several pads compounds .
    But the rear braking is not enough for me
    From 81 and newers model the rear master cylinder size is 14.29 and older model are 15.87.

    The travel of the pedal is like 1/3
    With the bigger old style I supose that the pedal travel will be more like 1/2
    somebody tried this or another brand rear master cylinder, i supose a smaller brake master cilinder will be a better idea.
    My k1200rs have the same brembo rear caliper, same compound pads and the braking is far superior. (I believe 13mm).
    My caliper is already rebuilt, new stainlesshose line
    Thanks in advance for your advice
    #1
  2. bmwrench

    bmwrench Long timer

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    Drill a new hole in the lever for the brake rod 6mm lower than the original. You may need to file the lever a bit to make this work. Make sure that you haven't eliminated free play when you reassemble it.
    #2
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  3. mattcfish

    mattcfish R90X Supporter

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    A smaller size will give easier braking but longer travel.
    #3
  4. Eduardocastrejon

    Eduardocastrejon Been here awhile

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    Thanks
    Do you have photos ?
    #4
  5. Eduardocastrejon

    Eduardocastrejon Been here awhile

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    Thanks, my mistake
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  6. Eduardocastrejon

    Eduardocastrejon Been here awhile

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    Something like this, I supose that will have faster action, i already ask for other master cylinder but i will try this

    Attached Files:

    #6
  7. bmwrench

    bmwrench Long timer

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    Drill the hole in the lever you step on, not the one at the master cylinder. What you've done will give you less power, not more.
    #7
  8. Eduardocastrejon

    Eduardocastrejon Been here awhile

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    Did you refer to brake handle lever ?
    #8
  9. bmwrench

    bmwrench Long timer

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    I guess I should have typed "brake pedal".
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  10. Eduardocastrejon

    Eduardocastrejon Been here awhile

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    Yes, my mistake , brake pedal
    Thanks

    at this place (green dot) or oposite side ?

    Attached Files:

    #10
  11. Eduardocastrejon

    Eduardocastrejon Been here awhile

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    Maybe here I have a problem
    I read that exist diferent brake pedal shapes
    I sent to polish mine and was changed for other bike
    My bike is 81 rear disc brake
    And the pedal that installed came from 80 with disc brake too ( ate at front -brembo at rear)
    #11
  12. bmwrench

    bmwrench Long timer

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    AFAIK, they're all the same. You should drill a little farther back than the green dot, and bit lower.
    #12
  13. Eduardocastrejon

    Eduardocastrejon Been here awhile

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    Thanks
    Yes, its an imorivement
    Less pedal travel, brakes works sooner
    I can apply more force
    thanks
    Still is not a good rear brake
    We need an improvent here
    I suppose another type of master cylinder is enough

    Attached Files:

    #13
  14. bmwrench

    bmwrench Long timer

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    Replace the rear disc.
    #14
  15. kcoralj

    kcoralj Been here awhile

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    Interesting, those are 9/16 and 5/8 inch.
    Strange they used those sizes for the rear and metric for the front.
    #15
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  16. spokester

    spokester Long timer

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    If you have noticed the bleeder bolt takes an 11mm wrench then that's really 7/16 inch. Thank NATO for that!
    #16
  17. bmwrench

    bmwrench Long timer

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    That always amused me; BMW apparently couldn't bear to admit that they were fractional. When you get a new one now, they're a true 14 and 15mm.
    #17
  18. kcoralj

    kcoralj Been here awhile

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    It's quite common with hydraulic parts, U.S manufacturers have switched to metric prints but there's still a mix of metric and SAE fittings and spool valves are still 5/8 and 3/4 they're just called out as 15.875 and 19.05 mm on the print.

    I can't imagine why they called it out that way, front brakes and rear brakes must have been separate engineering departments.
    #18
  19. Eduardocastrejon

    Eduardocastrejon Been here awhile

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    Aftermarket I guess like ebc or ferodo, are pricey !
    #19
  20. bmwrench

    bmwrench Long timer

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    I didn't think EBC worked as well as stock.

    I'm a little concerned that you want more rear brake. I barely use mine.
    #20