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DOT 4, DOT 5.1Brake fluid. Whats the deal?

The Pikey

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If you look at the best Castrol racing SRF its only Dot 4. Reason is that the now common Dot 5.1 requirement has a very low temperature component which apparently lowers the high temperature boiling point slightly. As you can't have a fluid that does both low and high temperature then this means that Dot 4 is better for motorsport use. Thats what the head of Castrol brake fluid told me a few years ago.
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Dot 5.1 certification has the highest temp spec. requirements of the usual Dot 3, 4, both wet and dry.


SRF is a sublty different chemical base to give way higher dry and wet boiling points than Dot 5.1 and certainly Dot 4. The reason it fails Dot 5.1 certification seems to be a failure of long term durability tests probably due to it's inscreased affininty for water compared to Dot 5.1 - hence the Dot 4 classfication. And of course, once it starts to absorb water, it's abilities fall rapidly - hence the need to change SRF regularly as recommended in the blurb with it.





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