Brake fluid, and why 5.1 isn't the best.

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  1. Brake fluid, and why 5.1 isn't the best.
    #1
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    You might have noticed that there are a number of different brake fluid types available, all classified with a "Department of Transportation" number (DOT, this is the US D.O.T , the UK is Department of Transport.

    There are 2 main classes, Silicone based,(DOT 5) and Glycol based. (DOT 3/4/5.1)

    Glycol based fluids will pull water out of the air, and reduce their boiling point, Silicone based fluids won't, but using Silicone in a system that has had other fluids in it can lead to water and moisture pooling in areas, which can boil and ruin your brakes.

    Glycol fluids mix with the water and hold it, it will reduce the effectiveness, but it'll be gone the next time it's changed.

    You're unlikely to see anything these days with DOT5 in it, likewise for DOT3 which has been replaced by DOT4.

    What can be mixed?
    Do not mix DOT5 with anything, DOT3, DOT4 and DOT 5.1 can be mixed with each other without problems, but it'll be as bad as the weakest link, which may be old damp fluid, or a lower rated fluid added into a better system.

    What are the differences?

    DOT3 -
    dry
    205 C
    wet
    140 C


    DOT4-
    dry
    230 C
    wet
    155 C


    DOT 5.1
    dry
    270 C
    wet
    190 C

    The important bit is these are the MINIMUM boiling points allowed to be in a certain spec.

    What's the best fluid then?

    If you are stuck, and have only a few options, then you'll probably find that a DOT 5.1 will be the best performing, but there are better fluids out there.


    ATE Super Blue ~ 20/litre
    dry
    280 C
    wet
    192 C

    Millers Racing Brake fluid ~ 14/500ml
    dry
    310 C
    wet
    194 C

    Motul RBF600 ~15/500ml
    dry
    312 C
    wet
    216 C

    Motul RBF660 ~20/500ml
    dry
    325 C
    wet
    204 C

    Castrol SRF ~40/litre
    dry
    310+ C
    wet
    270 C


    All of the above fluids are in fact DOT4s, and Castrol SRF is DOT 3/4. Castrol SRF doesn't show a boiling point in most documentation, just a statement that it's easily over 300C, and it's Wet boiling point is massive compared with most others. It will soak up water far quicker than any other the other types, and usually is used for racing applications where changes are far more frequent.

    It just goes to show that all of the best fluids are DOT4 rated, so be aware that getting a good DOT4 is always better than an average DOT 5.1
    You won't feel a difference between each type, that's all down to hydraulics, but for track work, boiling the fluid will ruin your day, and potentially be quite dangerous.

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  4. #2
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    good info dude!!

    Perfect timing as I am getting my breaded lines on soon

    thanks

  5. #3
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    I've tended to use Super blue, as it has a lovely colour (it is actually blue, so you can see it bleeding though to the calipers) and it's easy to get. Either Eurocarparts or GSF sell it. I forget which one, but it means you can often pop out and get something decent on a saturday.

    The rest are all available online in various places, Opie Oils stocks all of them apart from super blue, and a lot of them are better still, but usually more expensive. That's why i included price / size for a better comparison.

    I haven't had any problems with superblue as yet, but as you can see, there are even better ones out there.
    Prices are a rough guide, so shop around if you are buying on the 'net.

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  8. #4
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    Hi,

    Will be bleeding through my brakes next weekend as i bought some braided hosing.

    Trying to decide between Castrol SRF or RBF 600

    SRF is very pricey, i can't get it trade so have to pay full RRP can get the RBF cheap though

    Is SRF worth it ? I don't do track events (yet) but do subject the brakes to some heavy applications....
    E39 M inspired

  9. #5
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    not for that stuff, RBF is still pretty strong, and you'd need a lot of heavy braking to start boiling the fluid.

    SRF needs to get changed more often as well, so i wouldn't use it unless it's a dedicated track or race car.

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    Ok, thanks for the info.

    RBF require frequent changing ?
    E39 M inspired

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    The millers fluid is very good, however the ate stuff seems very good on price so i might give that a go next brake fluid change!

  12. #8
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    Is RBF 600 going to be ok in a 98 E39 523 ? I've no idea what the stock fluid is in them

    How much fluid am i going to need ? It comes in bottles of 500ml. I will be removing my calipers and fitted new hoses, hope to totally drain all the oil fluid out the system.
    E39 M inspired

  13. #9
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    Superblue is more than good enough, but RBF is a little better still.
    I normally plan for about a litre of brake fluid, although i don't really pay too much attention to how much i put in. I think 500mls would actually do it.

  14. #10
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    Hi,

    What i meant was in terms of it's type rather than its performance, im not sure what DOT is used in the car from stock (im guessing it's probably still stock!)

    I'm having to mail order it as i can't find any suppliers round here, don't want to not have enough will 500ml defo be enough ?
    E39 M inspired

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