[Healeys] Brake Fluid -- Silicone vs. glycol

[simon.lachlan at alexarevel.plus.com]

I was getting myself in a muddle over brake fluid once so I made myself some
notes. Probably pinched from Google; I don't recall:-

1)	All brake fluid is "synthetic".it's not a "natural" product.
2)	Most synthetic fluids are NOT silicone. They are polyethylene glycol
ether based.
3)	DOT 3 & 4, which can be mixed, are not silicone. They are glycol.
4)	DOT 5, which cannot be mixed with 3 or 4, is silicone. Can't be
mixed with anything!
5)	DOT 5.1 is glycol based and cannot be mixed with DOT 5.
6)	DOT 5.1 can be mixed with DOT 3 or 4, as both are glycol based.

The word "synthetic" began to appear when manufacturers worked out that it
made regular brake fluid sound more special ie they could charge us more
money for the same old stuff.

DOT stands for Dept. Of Transport and relates to the fluids' boiling points.
If a car brakes at high speed, the brakes get hot. Some heat is carried away
from the brakes by the brake fluid. Bad news if the fluid boils! The faster
you're going, the higher DOT you need. Higher DOT numbers indicate higher
boiling points for "Dry" fluid (has not absorbed any water) and "Wet" fluids
(has absorbed water). ie, the wetter it is, the more problematic it becomes.
Mixing 3 with 4 lowers the boiling points and vice verca.