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  1. #1
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    Juicy 7's and DOT 3 fluid...

    I have some Juicy 7's on my downhill bike.
    The rotors are 203mm rotors. The front needs bleeding, but my avid bleed kit was out of fluid. So I bought some brake fluid from the gas station and filled and degassed my syringes. Well the fluid I bought was DOT3. Will this harm my brake system if I use it to bleed my juicy 7's or should I clean the syringes out and get some dot4 from the Auto parts store? Will running DOT3 hurt anything or is the 4 better with higher temps?

  2. #2
    29 some of the time...
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    Should be just fine. The stuff I got last time didn't differentiate, it was called Dot3 Dot4 compatible.
    Quote Originally Posted by saturnine
    that's the stupidest idea this side of pinkbike.

  3. #3
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    Quote Originally Posted by AL29er
    Should be just fine. The stuff I got last time didn't differentiate, it was called Dot3 Dot4 compatible.
    That means it's DOT4.

  4. #4
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    Quote Originally Posted by Tigerstripe40
    I have some Juicy 7's on my downhill bike.
    The rotors are 203mm rotors. The front needs bleeding, but my avid bleed kit was out of fluid. So I bought some brake fluid from the gas station and filled and degassed my syringes. Well the fluid I bought was DOT3. Will this harm my brake system if I use it to bleed my juicy 7's or should I clean the syringes out and get some dot4 from the Auto parts store? Will running DOT3 hurt anything or is the 4 better with higher temps?
    Brake fluid requirements are not typically designed to be backwards-compatible. They are forwards-compatible. IE a brake system designed for DOT 4 can use DOT 4 or higher but not DOT 3 fluid.

    EDIT: You could probably get away with it but DOT 4 and 5 are better and why risk it? DOT 3 is notorious for water absorption.
    Last edited by cchase86; 05-07-2009 at 12:21 PM.

  5. #5
    Meh.
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    It will work with DOT 3, but I suggest buying a better fluid, especially if you're riding downhill.

  6. #6
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    Water absorption and corrosion
    The big bugaboo with DOT 3-4 fluids always cited by silicone fluid advocates is water absorption. DOT 3-4 glycol based fluids, just like ethylene glycol antifreezes, are readily miscible with water. Long term brake system water content tends to reach a maximum of about 3%, which is readily handled by the corrosion inhibitors in the brake fluid formulation. Since the inhibitors are gradually depleted as they do their job, glycol brake fluid, just like anti-freeze, needs to be changed periodically. Follow BMW's recommendations. DOT 5 fluids, not being water miscible, must rely on the silicone (with some corrosion inhibitors) as a barrier film to control corrosion. Water is not absorbed by silicone as in the case of DOT 3-4 fluids, and will remain as a separate globule sinking to the lowest point in the brake system, since it is more dense.

    Since DOT5 does not absorb water, any moisture in the hydraulic system will "puddle" in one place. This can cause localised corrosion in the hydraulics.

    Doesn't say it but the lowest point is usually your caliper. Water has a much lower boiling point then brake fluid. Race cars get away with it since they are always changing their fluid.

    DOT5.1 provides superior performance over the other brake fluids discussed here. It has a higher boiling point, either dry or wet, than DOT 3 or 4. In fact, its dry boiling point (about 275 degrees C) is almost as high as racing fluid (about 300 degrees C) and 5.1's wet boiling point (about 175 to 200 degrees C) is naturally much higher than racing's (about 145 C).
    DOT5.1 is said to be compatible with all rubber formulations.
    Disadvantages:

    DOT5.1 fluids are non-silicone fluids and will absorb water.
    DOT5.1 fluids, like DOT3 & DOT4 will eat paint.
    DOT 5.1 fluids are more difficult to find.
    DOT 5.1 will be more expensive than DOT3 or DOT4.

  7. #7
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    Quote Originally Posted by cchase86
    Brake fluid requirements are not typically designed to be backwards-compatible. They are forwards-compatible. IE a brake system designed for DOT 4 can use DOT 4 or higher but not DOT 3 fluid.

    EDIT: You could probably get away with it but DOT 4 and 5 are better and why risk it? DOT 3 is notorious for water absorption.
    Whats the problem with water absorption?

    The reason I ask is because I have a 1976 Toyota Land Cruiser that has had DOT3 in it since it rolled out of the factory. I have not fully changed out the fluid in the 9 years I have owned the rig. I think it would be safe to assume that it's had DOT3 fluid in the system for at least 20 years. None of the lines have rusted out.

    Also, considering that the major components of the brake system in a bicycle are aluminum and plastic (aluminum does indeed corrode but unless there is a dissimialr metal in there, I am not sure it's an issue). Is there any steel in the system to make me worry about galvanic corrosion?

    My only major concern is the heat produced. I have noticed brake fade due to heat with my Juicy 5's. However, this bike has larger rotors on it.

    I suppose I should just get a bottle of DOT 5.1 fluid and replace the fluid in the brakes entirely.

  8. #8
    Happy in Happy Valley
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    Besides corrosion, water absorption will significantly lower the boiling point of the brake fluid.
    Rigid Surly 1x1 650b--------Fixed CrossCheck--------Surly Pacer-------Salsa Ala Carte

  9. #9
    Meh.
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    A high quality DOT 4 (RBF600) or DOT 5.1 can increase the boiling point of the fluid and help prevent fade.

    Though if you're fading them out with 203mm rotors and a fresh bleed, you may need to reconsider your braking technique.

  10. #10
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    Quote Originally Posted by cchase86
    Brake fluid requirements are not typically designed to be backwards-compatible. They are forwards-compatible. IE a brake system designed for DOT 4 can use DOT 4 or higher but not DOT 3 fluid.

    EDIT: You could probably get away with it but DOT 4 and 5 are better and why risk it? DOT 3 is notorious for water absorption.
    Never, ever ever ever use DOT5 in anything that requires Dot 3 -4- or 5.1 fluid.
    DOT5 is a different fluid type and will ruin your brake seals.
    Now, DOT 5.1 is fine.

    All regular DOT fluids absorb water, they are hygroscopic. Meaning they absorb water.
    The main difference between the 3, 4 and 5.1 dot fluids is the heat resistance. 5.1 has a higher rating. At least that is how they market the stuff.
    I heard Jerk Chicken mention once that there was a specific brand of dot4 fluid that had a better rating than most 5.1 fluids......at least that is how I interpreted what he said. I forget what fluid it was though.

  11. #11
    ballbuster
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    Don't use...

    Quote Originally Posted by cchase86
    Brake fluid requirements are not typically designed to be backwards-compatible. They are forwards-compatible. IE a brake system designed for DOT 4 can use DOT 4 or higher but not DOT 3 fluid.

    EDIT: You could probably get away with it but DOT 4 and 5 are better and why risk it? DOT 3 is notorious for water absorption.
    .... DOT5. You can use DOT 5.1, but not DOT5. DOT5 is silicone based, and is not compatible. *edit* somebody beat me to it!!

    You can prolly get away wtih DOT3 in a pinch, but it will boil at a lower temperature. Just man up and get some DOT4. It's not that hard to find. Heck, my grocery store carries it.

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