Mixing DOT brake fluids.

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  • Mixing DOT brake fluids.
  • I have DOT 4 and can’t get 5.1. I have no idea what fluid I added to my brakes last time they needed a bleed. Does it matter if I used 5.1 last time and add a little 4?

    If it does matter, Can I simply flush the system through. I only need to top the reservoir up as I accidentally let a tiny bit too much out of the calliper last night as I wanted a little more retraction.

    They’re HOPE Mono M4s.


    Premier Icon weeksy

    Since DOT 4 and 5.1 are both glycol-based brake fluids they are compatible with each other,


    it’s dot 5 you cannot mix with others as it is silicone based.

    4 and 5.1 fine. 5.1 is not always better than 4 anyway.

    No problems.

    The DOT glycol brake fluid standards (3, 4, 5.1) specify minimum wet and dry boiling points. It’s not uncommon for some high performance varieties to be DOT4 because they have higher dry boiling points but don’t fare so well when mixed with water (but in those car’s you’d change the fluid more often so it’s moot).


    It’s an M4, 203mm floating rotor on my wife’s nice, light, communting Inbred. I don’t think the fluid boiling points make too much difference? 🙂

    Premier Icon Northwind

    You can’t mix anything with DOT5 but that doesn’t matter because nobody uses 5 anyway.

    There’s some interesting options, like Castrol React has a wet boiling point higher than most dry boiling points- so if you were planning to neglect your bike for 20 years it’d be an awesome choice.

    Why haven’t I heard of this before Northwind?

    Just bled this for the first time since they left the shop. I know that i’ve never touched the hub they’re attached to (14 years old) and it’s had 10+ cassettes.

    My favourite bike is (literally) polished after every off road ride. The others, well, I’ve turned neglect into an artform.

    Premier Icon Northwind

    It does cost £43 a bottle.

    (also, changing fluid reduces the risk of corrosion and gets the wee bits of degraded rubber and crap like that out)

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