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  1. #1
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    o-ring material question

    what are the two compounds from which the DOT and mineral oil compatible o-rings made from?

    i've conducted a little experiment: i took the slave piston o-rings from hayse (DOT) and magura (mineral oil), and put them in different fluids. the DOT o-ring swell in every fluid i used: mineral oil, bicycle degreaser, silicon based lubricant, some sort of alcohol mixture. the o-rings (i cut it to several pieces, for positive and negative control as well) swell in every fluid but the DOT fluid.
    when i repeated the experiment for the magura's o-ring, non of the fluids caused swelling.
    it is widely known that oil causes rubber to swell. i suspect that the mineral oil using brands, like magura and shimano, use a special kind of rubber, which makes it impervious to oil, and also to a wide variety of fluids. whereas a system which uses DOT fluid has simple rubber o-rings, making it very susceptible to the elements (might explain why avid's pistons are always sticky, even on new brakes).

    i'd like to know what are the two rubber compounds used in the two cases.
    thanks in advance

  2. #2
    Nouveau Retrogrouch SuperModerator
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    Lots of different materials are used for O rings
    http://www1.mscdirect.com/O-Rings/O-...000001926.HTML
    mtbtires.com
    The trouble with common sense is it is no longer common

  3. #3
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    I beleive EPDM is used for DOT fluid and nitrile for mineral oil.

    The main issue with DOT Fluid is that it is absorbed into the rubber, then when exposed to another fluid the DOT fluid tries to get out of the rubber and this then causes the swelling.....

    So if you put mineral oil in a set of DOT brakes that has never seen DOT fluid the seals would not swell.

    If you then removed the mineral oil and put DOT fluid in the seals may swell a little bit.

    on the other hand if you removed the DOT fluid and then added mineral oil the seals would swell badly.

  4. #4
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    thanks jeff, thats very interesting.
    why dont manufacturers use nitrile rubber for DOT systems as well?

  5. #5
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    Quote Originally Posted by brake jack
    thanks jeff, thats very interesting.
    why dont manufacturers use nitrile rubber for DOT systems as well?

    I believe that the DOT fluid is just too aggressive for the nitrile rubber, in other words too much DOT fluid is absorbed by the nitrile to have good stabile o-ring performance.

  6. #6
    Vaginatarian
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    could be why shimanos brakes leak in very cold weather

  7. #7
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    My experience

    I've been using a whole shimano (mineral oil) hose kit, banjo fitting, orings, banjo nut, hose on a Hope m4 (dot 5.1). I just wanted to ride and if it only lasted a day that would be alright because I got to ride my new frame that weekend. Figured i'd order the Hope kit when it would start to leak, been 5 years, zero issues.

  8. #8
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    Quote Originally Posted by Ratt
    I've been using a whole shimano (mineral oil) hose kit, banjo fitting, orings, banjo nut, hose on a Hope m4 (dot 5.1). I just wanted to ride and if it only lasted a day that would be alright because I got to ride my new frame that weekend. Figured i'd order the Hope kit when it would start to leak, been 5 years, zero issues.
    what fluid did you use?

  9. #9
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    Quote Originally Posted by brake jack
    what fluid did you use?
    dot 5.1

  10. #10
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    nitrile orings can be used as static seals ( non moving ) in a DOT system with some success, so long as the tolerances between parts are nice and tight. You'll find when you take the orings out they will have swollen and gone very soft.

    should you try to use them in a moving part of the system ( MC for example ) they will likey fail within hours of installation and/or use

  11. #11
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    Quote Originally Posted by slidecontrol
    nitrile orings can be used as static seals ( non moving ) in a DOT system with some success, so long as the tolerances between parts are nice and tight. You'll find when you take the orings out they will have swollen and gone very soft.

    should you try to use them in a moving part of the system ( MC for example ) they will likey fail within hours of installation and/or use
    how do you know that?

  12. #12
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    5 years building making seals for the hydraulic and automotive industry, 2 years prior to that doing sales and support for the same.

  13. #13
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    Quote Originally Posted by slidecontrol
    5 years building making seals for the hydraulic and automotive industry, 2 years prior to that doing sales and support for the same.
    damn you, and your bulletproof logic. oh well

  14. #14
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    http://www.efunda.com/designstandard...=Brake%20Fluid

    this page has a table showing typical elastomer compatibility with brake fluid ( DOT)
    Ratts experience backs up what I said about static sealing, by only sealing the hose and banjo parts, there are no moving ( dynamic ) nbr seals in his DOT system.
    I don't recommend it but you can get away with it.

    if you're really convinced that NBR and DOT fluid play nice, fill a shimano or magura brake with DOT fluid and try your luck. just make sure there's an ambulance at the bottom of that hill cos you'll need it.

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