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Brake Master Cylinder

This is a discussion on Brake Master Cylinder within the Technical forums, part of the Caferacer.net Forums category; Is it ok to replace a 14mm piston bore with a 12.7mm piston bore? Operates a single disc, single pot caliper on a 1980 CM400....

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  1. #1
    Senior Member spacmanspif300's Avatar
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    Brake Master Cylinder

    Is it ok to replace a 14mm piston bore with a 12.7mm piston bore?
    Operates a single disc, single pot caliper on a 1980 CM400.

  2. #2
    Member chelpdogg's Avatar
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    just squeeze harder . .
    the question has to be asked why you would be doing it?
    i'm assuming u mean replacing the whole master cylinder setup.
    if u get technical. surface area of piston is reduced by about 18% so guess u gotta squeeze that 18% harder to get same braking
    That\'d be a good idea if it made any sense

  3. #3
    Member chelpdogg's Avatar
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    just squeeze harder . .
    the question has to be asked why you would be doing it?
    i'm assuming u mean replacing the whole master cylinder setup.
    if u get technical. surface area of piston is reduced by about 18% so guess u gotta squeeze that 18% harder to get same braking
    That\'d be a good idea if it made any sense

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  5. #4
    Senior Member KeninIowa's Avatar
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    Actually it's smaller bore = more pressure/less volume on the MC. Opposite on the caliper. Their size relative to one another is called the leverage ratio.

    Cypher this out;

    http://vintagebrake.com/mastercylinder.htm

  6. #5
    Senior Member KeninIowa's Avatar
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    Actually it's smaller bore = more pressure/less volume on the MC. Opposite on the caliper. Their size relative to one another is called the leverage ratio.

    Cypher this out;

    http://vintagebrake.com/mastercylinder.htm

  7. #6
    Senior Member spacmanspif300's Avatar
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    Ratty,
    I appreciate the info, but I'm not convinced it is true or helpful.

    He suggests single piston calipers should be in the 14:1 to 21:1 range. I measured my brake cylinder diameter at 37mm. That combined with my stock 14mm master cylinder piston= approx 7:1 ratio, which according to that table would make my brake inoperable...this is not the case. A swap to 12.7mm (aka 1/2") master cylinder would make the ratio more like 8.5:1.

    Anyone who has ridden this bike will know that the disc is never too firm, has plenty of feel, and doesn't come close to locking. Of course, the CM front wheel is 19" (+tire makes it more like 24"), and the ~11" disc doesn't have great leverage against that. I guess what I'm saying is that this table seems kinda useless.

    Now if I had measured my brake pad surface area, we might be more in the 14:1 to 21:1 range he suggests. I'm probably just gonna buy the 12.7mm piston and see how it goes.


    Chelpdogg,
    Refrain from abbreviating three letter words, and I will consider whatever you have to say in the future.

  8. #7
    Senior Member spacmanspif300's Avatar
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    Ratty,
    I appreciate the info, but I'm not convinced it is true or helpful.

    He suggests single piston calipers should be in the 14:1 to 21:1 range. I measured my brake cylinder diameter at 37mm. That combined with my stock 14mm master cylinder piston= approx 7:1 ratio, which according to that table would make my brake inoperable...this is not the case. A swap to 12.7mm (aka 1/2") master cylinder would make the ratio more like 8.5:1.

    Anyone who has ridden this bike will know that the disc is never too firm, has plenty of feel, and doesn't come close to locking. Of course, the CM front wheel is 19" (+tire makes it more like 24"), and the ~11" disc doesn't have great leverage against that. I guess what I'm saying is that this table seems kinda useless.

    Now if I had measured my brake pad surface area, we might be more in the 14:1 to 21:1 range he suggests. I'm probably just gonna buy the 12.7mm piston and see how it goes.


    Chelpdogg,
    Refrain from abbreviating three letter words, and I will consider whatever you have to say in the future.

  9. #8
    Senior Member txchainsawgogi's Avatar
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    "Chelpdogg Refrain from abbreviating three letter words, and I will consider whatever you have to say in the future"

    Lulz.

    I'd say buy what you're planning on getting and test it out. Anything would be better than the garbage front brakes we have on our bikes!
    honk if youre horny.

  10. #9
    Senior Member txchainsawgogi's Avatar
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    "Chelpdogg Refrain from abbreviating three letter words, and I will consider whatever you have to say in the future"

    Lulz.

    I'd say buy what you're planning on getting and test it out. Anything would be better than the garbage front brakes we have on our bikes!
    honk if youre horny.

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