Brake Fluid explained

Discussion in 'Opie Oils' started by oilman, Nov 26, 2010.

  1. 26 Nov 2010 at 11:41 AM #1
    oilman

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    Brake fluid... Bit of a mystery topic!

    To help dispel some myths and for some good solid general info on the mysterious world of brake fluids I decided to contact Millers Oils up in West Yorkshire.

    Their Technical Director, Martyn Mann was on hand to give us some useful info… below is Martyn's article on brake fluids.

    There is a degree of confusion regarding the specification of brake fluid and this article sets out to clarify the situation.

    The Department of Transportation (DOT) classifies brake fluids to defined specifications. These specifications relate to their boiling points and chemical composition, both of which are important. All currently available brake fluids are covered by one of the following specifications; DOT3, DOT4, DOT5 and DOT5.1.

    The laws of thermo-dynamics dictate that the energy from motion is turned into heat through friction. A braking system only works efficiently if the fluid remains incompressible. If the brake fluid boils, it turns to gas, which is compressible and the braking system becomes “spongy” or in extreme cases fails completely.

    A brake system is not perfectly sealed and moisture can get into the system and be absorbed by the fluid. The effect is to reduce the boiling point of the fluid, which reduces the efficiency of the braking system, as described above.

    The DOT specifies two reference tests for brake fluids.

    * Dry boiling point - the boiling point of fresh fluid

    * Wet boiling point –the boiling point once the fluid has absorbed moisture (representing brake fluid after time spent in a real situation).

    There are two main types of brake fluids:

    * DOT 3, DOT 4, Super DOT4* and DOT 5.1 which are based on poly glycol compounds.

    * DOT 5, which are based on Silicone.

    Note the two types of fluid are not compatible and must not be mixed in a braking system.

    SILICONE BRAKE FLUID (DOT 5)

    Silicone based DOT 5 was originally introduced to give higher temperature performance over glycol DOT 4. Silicone fluid also has other advantages, it does not damage paintwork and it does not absorb water.

    However, silicone fluid is a poor lubricant and does not lubricate ABS pumps as well as PAG fluids. It is also more compressible than PAG fluids, which can result in a sluggish or spongy pedal. It therefore requires special design considerations in braking systems. Further, because it does not absorb water, any water remains as globules, which can pool in low spots in the system and cause corrosion. This water can vaporise when heated under heavy braking giving a disastrous effect on braking efficiency.

    DOT5 fluids are not recommended for motor sport applications.

    POLY GLYCOL BRAKE FLUIDS (DOT 3, 4 AND 5.1)

    Glycol based DOT 4 fluid is the current mainstream brake fluid, and you will see that the specification is considerably better than DOT 3 which it replaces.

    DOT 5.1 has higher specification still and is for fast road and occasional track day use. It has a similar spec to DOT4 for the boiling point (>260) but is a lot lower viscosity @-40C typically 900 centistokes (compared to 1500 - 1800 centistokes for DOT 4 and super DOT 4).

    Listed in the table below, are the minimum dry/wet boiling point specifications for each DOT level.

    BOILING POINT:
    DOT 3 - 205°C (dry) / 140°C (wet)
    DOT 4 - 230°C (dry) / 155°C (wet)
    DOT 5 (silicone) - 260°C (dry) / 185°C (wet)
    DOT 5.1 (PAG) - 260°C (dry) / 185°C (wet)
    Super Dot4 * - 300°C (dry) / 195°C (wet)
    (racing brake fluid)

    * Super DOT4: The main difference between DOT 4 and Super DOT 4 is the dry boiling point. Normal Dot4 is >260C whilst Super DOT 4 is more like>310C

    With thanks to Martyn Mann - Technical Director Millers Oils.

    Cheers

    Guy.

    Opie Oils
     
  2. 26 Nov 2010 at 7:01 PM #2
    paul46

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    nice write up and great info, i have always run a castrol super dot 4 in my race bike. great stuff
     
  3. 26 Nov 2010 at 8:43 PM #3
    oilman

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    That's a good fluid for the money, although for track use in cars, sometimes one of the higher spec Super DOT4s are necessary.

    Cheers

    Tim
     
  4. 21 Mar 2011 at 3:24 PM #4
    daniel782

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    TIM What brake fluid would you recommend for my E46 M3 Cab??

    I've got uprated Stoptech Discs and pads and Goodridge S/Steel Hoses for fast road, but not much track use.

    Thanks
    Dan
     
  5. 21 Mar 2011 at 3:58 PM #5
    JohnH79

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    I need to buy some brake fluid for my BBK. Not sure whether to get the Super Dot 4 or the Dot 5.1. Fast road use and occasional track days. What would you advise?
     
  6. 21 Mar 2011 at 4:54 PM #6
    oilman

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  7. 21 Mar 2011 at 5:14 PM #7
    daniel782

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    Thanks, just ordered the Fuchs Pro Race
     
  8. 22 Mar 2011 at 3:26 PM #8
    JohnH79

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    Hi Tim, is it just as cheap to order through the website or to order with you direct?

    I'd like the RBF600 brake fluid. Would 1 litre be enough to replace all the brake fluid front/rear or will I need 2L ?

    Regards

    John
     
  9. 23 Mar 2011 at 9:18 PM #9
    newm3luva

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    Hi, what is recommended for standard setup?
    Is castrol response DOT4 ok to use? Also how much is needed for a full bleed and refil? Is it 2.8litres?
     
  10. 29 Mar 2011 at 7:39 AM #10
    adz2k10

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    ordered
     
  11. 23 Jul 2012 at 7:06 AM #11
    RyanC

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    Did you find the answer to this mate?
     
  12. 23 Jul 2012 at 11:28 AM #12
    oilman

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  13. 15 Feb 2014 at 11:51 AM #13
    SwiftyM3

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    Can you pm me a cutters price :D please on the RBF 600 brake fluid. Correct me if I'm wrong but I think I need 3 liters to replace my old fluid.
     
  14. 17 Feb 2014 at 9:27 AM #14
    oilman

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  15. 10 Jun 2014 at 3:19 PM #15
    Adil

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  16. 10 Jun 2014 at 3:33 PM #16
    oilman

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    Hi

    That's ideal for an M3. I don't have the capacity listed, but I would think 1-1.5L will cover it.

    We don't have 20% off all fluids at the moment, but you can use the code M3C for 10% off everything.

    Cheers

    Tim
     
  17. 10 Jun 2014 at 3:48 PM #17
    Adil

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    Great,

    Thanks for the quick reply Tim. Will order up.
     
  18. 21 Jul 2015 at 10:08 AM #18
    Dalgib

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    I know this is an old thread, but currently what's the best brake fluid ( fast rd use) for my e46 m3 also is 1.5 ltrs what I need?
     
  19. 21 Jul 2015 at 10:46 AM #19
    oilman

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    The best fluids for the brakes and clutch are the Castrol SRF, Motul RBF660 and Gulf RF1000. The Motul RBF600, Gulf RF800, Millers 300 Plus and Fuchs Pro Race are close to those, but don't have quite as high boiling points. Those are overkill for road use, but can be used on road if you want. The next step down (in performance terms), but still an upgrade over standard are the Gulf Racing 5.1, Motul DOT 5.1 and Castrol React Performance. Any of the other DOT4 fluids we have are fine to use as a standard choice.

    http://www.opieoils.co.uk/c-450-brake-fluid-clutch-fluid.aspx

    Yes, a litre and a half should be plenty.

    Cheers

    Tim
     
  20. 21 Jul 2015 at 1:16 PM #20
    Dalgib

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    Ordered with yourselves thank you for your help
     

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