Re: Brake Fluid
I've seen some fluid Q's tossed around and thought I would share some of my understanding on brake fluid in case anyone is interested.
DOT 3, 4, & 5.1 are all Glycol based, so they will all eat your paint...however, this doesn't expressly mean they are interchangeable. The likelihood of having an issue caused by mixing them is relatively low, but since we're talking about performance and pushing brakes to their limit, we should observe the specs...and that's to say that different grades shouldn't be mixed.
Another good / bad about being glycol based they absorb H2O. The good is that you won't end up with potentially corrosive pockets of water in your brake fluid, the bad is that the more water it absorbs, the lower your boiling point gets.
Boiling point ranges:
Dry boiling point Wet boiling point
DOT 3 205 °C (401 °F) 140 °C (284 °F)
DOT 4 230 °C (446 °F) 155 °C (311 °F)
DOT 5 260 °C (500 °F) 180 °C (356 °F)
DOT 5.1 260 °C (500 °F) 180 °C (356 °F)
"Wet boiling point" is defined as having 3.7% water by volume rather than being pure fluid. So in the typical 500ml brake system, the addition of 18.5ml of water reduces the boiling point considerably. That being said; a teaspoon is *roughly* 5ml...so you're talking about 4 teaspoons of water in your brake lines.
DOT 5.0 unlike the others above is silicone based; b/c of this it doesn't absorb water, so the boiling point remains constant, buuut could lead to isolated pure water pockets in your fluid if exposure were to occur.
Combining glycol fluid with silicone fluid is no bueno. I've heard rumor that it can turn your brake fluid into jelly after intense heat exposure...this, I can't prove. But I can say for sure that there's a good chance you'll FUBAR your seals.
As for the big deal with boiling points; after your fluid reaches its BP due to heavy braking exposure, it turns from a purposeful liquid into a useless gas, and loses it's hydraulic properties. With that comes a lack of pressure and useless brakes, since they're not designed to work as a pneumatic system.