RWL writes:

BTW, what's in brake fluid?

Various glycols and glycol ethers. You want incompressibility for
hydraulic performance, low vapor pressure to avoid evaporation losses, high
boiling point to handle hot brake systems, miscibility with water to
disperse water contamination instead of separating it where it could freeze
or boil, non-corrosive to metal, inert to elastomers, low flammability.