What Brake Fluid?

I came to work this morning knowing I needed brake fluid. I was going to order some DOT5, simple! The higher the DOT number the better right? Well if you look at this table of DOT brake fluid standards you'd be forgiven for thinking the same:

But the table above doesn't tell you the full story. Let's have a look at another table;

As you can see the DOT standards also refer to what the fluid is made out of. The boiling points listed are the minimum required to meet those DOT standards.

What you need to know is that Glycol-ether is hygroscopic meaning its water absorbing. It will absorb water from the atmosphere as soon as you unscrew the lid off your shiny new brake fluid bottle. The wet boiling point ratings are based on 3.7% water content. I have seen it said that it takes 2 years for fluid to absorb 3% water but this figure increases when under heavy use, motorsport use!

DOT5 is made of silicone so its hydrophobic, it does not absorb water. However because it doesn't absorb water any water that finds its way into the system will remain as water 'globules", long term this can cause spongy brakes and in some cases corrosion.

DOT5.1 is recommended for cars with ABS and ESP due to its "constant viscosity under a wide range of temperatures"

NOTE: You must never mix DOT5 silicone brake fluid with any of the other Glycol based brake fluids. 

Now let's look at some boiling points for some popular high spec brake fluids:

Conclusion

I am by no means an expert on brake fluid, all of the above information was gathered by searching and reading on the internet, forums, product descriptions, chats with brake fluid manufacturers and Wikipedia.

I did find some interesting discussion on ATE Super Blue (or Type 200) vs Castrol SRF brake fluid. They are both highly rated by motorsport competitors but have very different price points. The advice in choosing was that if you like to change your brake fluid regularly then ATE Super Blue is very cost effective. But it you really don't change your brake fluid often then the Castrol SRF, because of its excellent wet boiling point rating, can also be very cost effective despite its much higher price.

I think I'll be taking home some ATE Super Blue for my own car as it's such a bargain but I think we'll be topping up the Motamec Escort project with the Castrol SRF. Please feel free to share your experiences and opinions in the comments section below.