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  1. #1
    Uncle
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    What to believe? DOT fluid confusion

    http://www.advancepetro.com/differentbrakefluid.htm

    According to this link, DOTs 3,4 & 5.1 are all hydroscopic, or absorb atmospheric moisture. This goes against what several MTBRers have stated in several threads here, where some responses have suggested that 5.1 is the non-hydroscopic (hydrophilic?) version of DOT 4.

    I have a 1/2 bottle of Motul DOT4 that's been sitting in my storage unit for ~2 years now, and from what I remember from my auto industry days, an open container of hydroscopic fluid shouldn't be shelved for long because it will absorb water while sitting. I've recently bled my Hopes from this bottle & already they're not working very well again. I'll be rebuilding these brakes soon, but when it's time to bleed them, I want to know if I should be skeptical of my fluid's abilities to do its job. Is it time for a bottle of 5.1?
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  2. #2
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    ALL dot 3-5.1 brake fluids absorb moisture by design. they're intended to do so, and its a good thing. free moisture in a brake system causes rust and very very rapid failure. a hydroscopic fluid prevents that free moisture.

    the 3-5.1 rating relates to boiling point.. nothing more, nothing less.

  3. #3
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    It's hygroscopic - with a 'g'.

    Here's a short article about DOT fluid, boiling points and shelf life. According to Castrol..."Shelf life is no longer than 2 years, provided the oil has been stored in a cool, dry area, with no extreme fluctuating temperatures and the seal has remained intact."
    Last edited by SteveUK; 09-28-2008 at 07:35 AM.
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  4. #4
    TLL
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    Quote Originally Posted by 105millimetersofpleasure
    http://www.advancepetro.com/differentbrakefluid.htm

    According to this link, DOTs 3,4 & 5.1 are all hydroscopic, or absorb atmospheric moisture. This goes against what several MTBRers have stated in several threads here, where some responses have suggested that 5.1 is the non-hydroscopic (hydrophilic?) version of DOT 4.

    I have a 1/2 bottle of Motul DOT4 that's been sitting in my storage unit for ~2 years now, and from what I remember from my auto industry days, an open container of hydroscopic fluid shouldn't be shelved for long because it will absorb water while sitting. I've recently bled my Hopes from this bottle & already they're not working very well again. I'll be rebuilding these brakes soon, but when it's time to bleed them, I want to know if I should be skeptical of my fluid's abilities to do its job. Is it time for a bottle of 5.1?
    It's time for a new bottle of 4, or whatever you prefer. You are right about shelf life on DOT fluid. I keep opened containers of DOT for 6 months, max. I really like the Motul fluid, BTW. Also a former car guy.
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  5. #5
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    DOT 5 is the only "different" silicone based fluid. You don't want moisure in any of them but DOT5 is less tolerant than the rest. Moisture will make the fluid go bad sooner and will make the brakes fade at a lower temperature. I think many people are getting 5 and 5.1 mixed up. Go with the 5.1, it has the highest boiling point, can be mixed with the others (except 5) and is a bit thinner than the rest.

  6. #6
    Uncle
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    Right on, Brohams! Thanks for clarifying the mystery. I will rebuild and bleed with the DOT 5.1 at my earliest convenience.
    thanks
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  7. #7
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    If you can find it, Motul RBF 600 is my favorite brake fluid (synthetic DOT4 Racing Brake Fluid with outrageously high boiling points - 593F dry, 420F wet). I get mine at the local motorcycle shop.

  8. #8
    Uncle
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    Great suggestion

    Quote Originally Posted by watermoccasin
    If you can find it, Motul RBF 600 is my favorite brake fluid (synthetic DOT4 Racing Brake Fluid with outrageously high boiling points - 593F dry, 420F wet). I get mine at the local motorcycle shop.
    I'll look for this stuff next time I'm due. My new Hope bleed kit came with a small bottle of 5.1, but based on how little will fit into the system, the small bottle makes plenty of sense.
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  9. #9
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    I've had great luck with ATE blue, and ATE gold. If you switch back and forth between the two - the color change lets you know when you've hit new fluid. To keep from wasting fluid by storing it on the shelves - bleed more then one system at a time (do the car first, then the bike)...

  10. #10
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    Quote Originally Posted by SSearchVT
    I've had great luck with ATE blue, and ATE gold. If you switch back and forth between the two - the color change lets you know when you've hit new fluid. To keep from wasting fluid by storing it on the shelves - bleed more then one system at a time (do the car first, then the bike)...
    ATE and Motul are both well regarded. Folks seem to like the Valvoline synthetic stuff too. It doesn't perform quite as well, but it's cheaper and much easier to come by. Pretty much any auto parts store will have it on the shelf.

  11. #11
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    Quote Originally Posted by wreckedrex
    ATE and Motul are both well regarded. Folks seem to like the Valvoline synthetic stuff too. It doesn't perform quite as well, but it's cheaper and much easier to come by. Pretty much any auto parts store will have it on the shelf.
    There is no such thing as a non-synthetic DOT brake fluid. Valvoline synthetic is nothing more than marketing at it's worst.

  12. #12
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    Quote Originally Posted by BuickGN
    There is no such thing as a non-synthetic DOT brake fluid. Valvoline synthetic is nothing more than marketing at it's worst.
    By "synthetic stuff" I guess I meant "Synpower." Sorry, couldn't recall their exact marketing jargon. In spite of the "marketing at it's worst" bit the stuff has a pretty decent boiling point. I guess it's not really any cheaper than super blue though, just much easier to get and comes in more realistic volumes for a bicycle... linky

  13. #13
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    Quote Originally Posted by wreckedrex
    By "synthetic stuff" I guess I meant "Synpower." Sorry, couldn't recall their exact marketing jargon. In spite of the "marketing at it's worst" bit the stuff has a pretty decent boiling point. I guess it's not really any cheaper than super blue though, just much easier to get and comes in more realistic volumes for a bicycle... linky
    You're right, it does say "synthetic" on the bottle and it is a good product. I just don't like the way they try and fool people.

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