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  1. #1
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    Dot 5.1 water absorption speed?

    I read that DOT 5.1 absorbs water more quickly than 3 and 4. Supposedly 5.1 is just a "race" fluid and 4 is better for recreational riders.

    It sounds like baloney to me.

    So, is this true? If so, given that 5.1 has a higher wet boiling temp, does it really matter in practice?

  2. #2
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    5.1 brake fluid are the ones that doesn't absorb water. most are silicon base so water just don't mix with it. this types of brake fluids are what is used on high end cars with brembo brakes and braided llines.

  3. #3
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    Quote Originally Posted by xristianos
    5.1 brake fluid are the ones that doesn't absorb water. most are silicon base so water just don't mix with it. this types of brake fluids are what is used on high end cars with brembo brakes and braided llines.
    Grossly incorrect. There are LMA types, even at least two commonly available DOT4 fluids, that have lower moisture absorption than some DOT 5.1's. 5.1 is glycol type and it will certainly absorb water. There is no 5.1 that is silicone based.

    They can all be formulated a certain way. 5.1 tends to be thinner, supposedly, to assist with the operation of ABS, TRAC, and stability functions of the ABS system. However, the ratings are all over the place.

    ATE, Valvoline, and Castrol all make LMA, high boiling point DOT4 fluids.

    Additionally, 5.1 is not Silicone fluid. DOT 5 is silicone. Please gather your facts before posting such a thing. This constantly gets debunked and only serves to show how oblivious some people are to what could be a glut of valid information out there. The only reason it has the "5" rating and then 5.1 went back to a glycol type is because the 5 simply came out and was rated in series.

  4. #4
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    Quote Originally Posted by xristianos
    5.1 brake fluid are the ones that doesn't absorb water. most are silicon base so water just don't mix with it. this types of brake fluids are what is used on high end cars with brembo brakes and braided llines.
    J_C has already called xristianos on this one, but worth emphasising that everything important in this post is wrong.

    DOT 5 is a fluid standard based on silicone. DOT 5 is never used on any serious performance car. Its lack of water absorption does not make up for its other inherently poor properties for a high performance braking system. However, rather than just hatin' on DOT 5 I'll point out that I specify DOT 5 fluid for clutch actuation on the race car clutch system that I designed and sell.

    DOT 5.1 is a fluid standard based on glycol chemicals with higher boiling points than DOT4 and therefore with most of the same advantages and shortcomings.

    If you want to see how fast DOT 5.1 absorbs water, leave the top off a half empty bottle and in any modestly humid environment you will see a rise in fluid level after a few days. Some fluids do vary in their water absorption while conforming to the same DOT 5.1 specification as other fluids.

  5. #5
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    I live in CO so finding a modestly humid environment will be a bit difficult.
    What would the worst case scenario be?

    Let's say I lived on the east coast, like Georgia, and it was a very humid environment.

    If I rode my bike 10 hours per week, and used DOT 5.1, how much more often would I have to flush my brake system compared to using a high quality DOT 4?

  6. #6
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    Quote Originally Posted by nato_the_greato
    I live in CO so finding a modestly humid environment will be a bit difficult.
    What would the worst case scenario be?

    Let's say I lived on the east coast, like Georgia, and it was a very humid environment.

    If I rode my bike 10 hours per week, and used DOT 5.1, how much more often would I have to flush my brake system compared to using a high quality DOT 4?
    Quote Originally Posted by Jerk_Chicken
    They can all be formulated a certain way. 5.1 tends to be thinner, supposedly, to assist with the operation of ABS, TRAC, and stability functions of the ABS system. However, the ratings are all over the place.
    Your question has too many variables. I quoted JC but want to add that the there is a MINIMUM dry/wet boiling point that a fluid must achieve to get a particular rating. For example Ford makes a heavy duty brake fluid that has a higher dry BP than the DOT 3 standard it is rated at, but it has a wet BP that only meets DOT 3 standards. Most fluids exceed their minimum standards and there are some "long life" and "high performance" fluids that exceed them by a considerable margin. In testing, each brand is going to have different ACTUAL dry/wet boiling points and I believe this is what JC refers to when he says the ratings are all over the place.
    holy...

  7. #7
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    yap. the'yre right. i assumed that being above dot 5 that it is based on silicone.

    Silicone Brake Fluids
    In years past, all brake fluids were glycol. Then D.O.T. 5, a silicone fluid having a higher temperature rating, emerged, initially to meet the higher boiling point requirements of racing use. (Race car brake systems include oil-cooler-like heat exchangers and ceramic pads.) Silicone fluid was able to withstand the most heat of any brake fluid, so it earned a reputation as a racing brake fluid. However, silicone brake fluid has properties very different from glycol fluid, and has its own pros and cons. On the advantage side, silicone fluid will not harm paint or plastic, and does not aggressively attract additional moisture as glycol fluid does. On the disadvantage side however, silicone fluid aerates easily. Harley-Davison, one of the sole current OEM users of silicone fluid, warns buyers to let the fluid sit at least an hour before using it. The trip home in the saddlebag is enough to aerate silicone brake fluid until it looks like a freshly poured soft drink. Silicone fluid is also slightly more compressible than glycol fluid, does not change color to tip the user to its moisture content, and worst of all, neither accepts or disperses moisture, making systems using it more corrosion prone, and requiring much more frequent fluid changes. Silicone brake fluid also lacks glycol fluid's naturally occuring lubricity, making it incompatible with the mechanical valving in some antilock braking systems.

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