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

[The Pikey]

I have always used DOT 5.1 high performance brake fluid from Halfrauds but on the master cylinder theres a label saying DOT 4 only. On the 5.1 bottle it says it mixes with all sorts of other fluid apart from silicon fluid. Am I ok to use the 5.1?

 

Jason

 

 

[Bricol]

Yes.

 

Silicon fluid is Dot 5.0 and doesn't mix with the others.

 

Bri

[Graham Perry]

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.

[Bricol]

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.

 

Bri