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  1. #1
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    DOT fluid and standard O-rings

    So I need to replace an o-ring in the lever of my formulas (they have swollen up)
    Do I need special DOT (5.1) resistant o-rings, or will standard plumbing o-rings resist the fluid corrosion?
    Thanks
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  2. #2
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    Nobody has any experience with this?
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  3. #3
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    Standard o-rings will degrade very quickly in DOT fluid, as it is highly corrosive. I'd recommend looking for a replacement from the manufacturer or a good bike shop. Many bike shops will have tackle boxes full of specific parts for hydraulic brake systems.

  4. #4
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    Cheers, i don't really have any local bike shops that are very specialised, but i'll try them. If not it looks like i'll just get some standard o rings, as that's better than leaking brakes!
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  5. #5
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    Quote Originally Posted by hartwerks View Post
    Standard o-rings will degrade very quickly in DOT fluid, as it is highly corrosive. I'd recommend looking for a replacement from the manufacturer or a good bike shop. Many bike shops will have tackle boxes full of specific parts for hydraulic brake systems.
    its not quite that simple. DOT brake fluid is glycol based and not actually corrosive - it does not harm metal in any way - but it does react with certain materials, for example it damages paint. Using an incompatible fluid will also not actually dissolve O-rings (well, unless it is something really evil). The problem is that the O-rings might swell up.

    Here is a compatibility chart from an O-ring manufacturer, Glycol brake fluid is listed under "brake fluid DOT3" which is basically the same stuff as DOT4 and 5.1.
    Marco Rubber | Fluid Compatibility Guide

    If the O-rings he got are Nitrile Rubber (Buna-N), it's a no-go. If they are EPDM (see "Ethylene Propylene" in the chart), they're fine - in fact that is what DOT brakes come with.

    To put that in perspective, have a look at the in the chart for mineral oil, which is used in many other brakes.


    People always think mineral oil = harmless and DOT Brake fluid = evil, not the case at all. Mineral Oil will also damage certain materials, just different ones. Natural Rubber O-rings are not a good idea with mineral oil because they swell. Mineral oil is also incompatible with EPDM seals (which to my knowledge is one of the main reasons that some brake manufacturers choose DOT fluid over mineral oil).


    Lesson: There are many, many different types of O-rings, and each will react different to a given fluid. Make sure you get the right one, or stick with whatever the brakes came with.
    Last edited by lelebebbel; 09-18-2012 at 02:31 AM. Reason: original links wouldn't work, changed links
    wanted: Cannondale Lefty w/ V-brake studs

  6. #6
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    Thanks so much dude! I really appreciate that answer! I'll take the list of compatible materials and see if he has any of them :P cheers
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  7. #7
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    Daayyum. Somebody hates me :/ Just realised I have -18 rep points?!! I sware it was positive until now? :S
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  8. #8
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    Thanks lelebebbel.

    I was tempted to use DOT 5.1 since it doesn't absorb water like 3 and 4.

  9. #9
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    DOT 5.1 is hygroscopic just like DOT3 and DOT4. This is a feature, not a problem. It is meant to absorb water to prevent water bubbles from forming in your brakes (although this is something that you typically wouldn't have to worry about on a bike anyways, it's more of a problem for a car with its long brake lines and big reservoirs).
    The difference between DOT5.1 and DOT4/3 is that it maintains a higher boiling point even after it has absorbed some water.

    DOT 5 is not hygroscopic, but it is not compatible with DOT 3/4/5.1 brake systems and generally very rare.

    DOT 3/4/5.1 are glycol fluids, DOT 5 is a silicone based fluid.
    wanted: Cannondale Lefty w/ V-brake studs

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