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  1. #1
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    Mixing DOT 4 and DOT 5.1 fluid?

    Just got back from vacation and tried to go for a ride, but was stopped when I found that my rear brake needs to be bled, badly. I'm all out of the Avid DOT fluid (which I assume was DOT 5.1 fluid) that came with my bleed kit for my Juicy Ultimates. The shops around here don't stock just brake fluid, but I was told I could go to an automotive store to get some.

    However, the guy in the shop had never heard of DOT 5.1 fluid. The manual says that the Juicys can work with DOT 4 fluid though, so I got a bottle of that. But the brakes still have some DOT 5.1 fluid in them... is it okay to add the DOT 4 fluid and mix the two inside the brakes?

    I would assume this is okay... as it seems like the only reason for the specific type of brake fluid is that is it has to be something that can withstand the high temperatures and thus if both liquids are both heat-resistant enough mixing them shouldn't be a problem... am I on the right track here?
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  2. #2
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    You're on the right track, yes. You can mix 4 and 5.1, but, in very loose terms, the resulting mixture will only be as good as the lesser fluid. With this in mind, the general rule is to flush all of the old fluid out. For most riders, though, the effects of mixing would probably be unnoticeable.
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  3. #3
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    Both are polyethylene-glycol based...5.1 has a higher boiling point than DOT 4. Mixing them will not harm your system, only lower your boiling point which may affect braking performance if put under high temp situations (like full Sintered Metallic pads on a long DH run). For most situations, you would never notice a difference.

  4. #4
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    Quote Originally Posted by mtnbiker72
    Both are polyethylene-glycol based...5.1 has a higher boiling point than DOT 4. Mixing them will not harm your system, only lower your boiling point which may affect braking performance if put under high temp situations (like full Sintered Metallic pads on a long DH run). For most situations, you would never notice a difference.
    That is a myth. There are 4's well higher than 5.1's. Just because a brake fluid has a higher DOT number, it doesn't immediately mean it has a higher BP. There are other parameters brake fluids are designed for, such as operating in ABS/Traction/Stability control systems and free flow through small channels.

  5. #5
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    You will have better luck at a motorcycle store to get dot 5.1. Dot 4.0 is usually avail at auto parts stores. The motorcycle store will be cheaper by a lot compared to the bike shop.

    Also, get as small of a container as you can. It does not store that long once it's open. It's better to have two smaller containers than one big one.

    I store mine in this collapsible bottle that I had. It minimizes the air in the bottle at when stored and makes it easy to suck the fluid in the syringe.
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  6. #6
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    Quote Originally Posted by Jerk_Chicken
    That is a myth. There are 4's well higher than 5.1's. Just because a brake fluid has a higher DOT number, it doesn't immediately mean it has a higher BP. There are other parameters brake fluids are designed for, such as operating in ABS/Traction/Stability control systems and free flow through small channels.
    Allow me to clarify...the MINIMUM boiling point requirements are higher for DOT 5.1 than DOT 4

  7. #7
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    Some dot4 oils have higher MBP's higher than the MBP's of many DOT 5.1's.

    The minimum "requirement" is hardly reflective of reality.

  8. #8
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    Quote Originally Posted by Jerk_Chicken
    Some dot4 oils have higher MBP's higher than the MBP's of many DOT 5.1's.

    The minimum "requirement" is hardly reflective of reality.
    The minimum boiling point is important to the average mountain bike rider who is looking to bleed their brakes. Most are not going to spend the time researching which DOT 4 has a higher boiling point than which DOT 5.1...so knowing that the minimum boiling point of DOT 5.1 will meet their needs REGARDLESS of whether or not another product has a higher boiling point is very reflective of reality

  9. #9
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    Both are polyethylene-glycol based...5.1 has a higher boiling point than DOT 4. Mixing them will not harm your system, only lower your boiling point which may affect braking performance if put under high temp situations (like full Sintered Metallic pads on a long DH run). For most situations, you would never notice a difference.
    Thanks! That's kinda what I was thinking.

    You will have better luck at a motorcycle store to get dot 5.1. Dot 4.0 is usually avail at auto parts stores. The motorcycle store will be cheaper by a lot compared to the bike shop.

    Also, get as small of a container as you can. It does not store that long once it's open. It's better to have two smaller containers than one big one.

    I store mine in this collapsible bottle that I had. It minimizes the air in the bottle at when stored and makes it easy to suck the fluid in the syringe.
    Yeah, I got the smallest bottle they had. It was still a lot bigger then I needed... but only cost about $3. Lots cheaper then the bike shop. Thanks for the tip about the motorcycle stores too.
    Speed has never killed anybody. Suddenly becoming stationary... that's what gets you.

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