For example, why couldn't brake fluid be used for transmissions or vice versa or power steering fluid as brake fluid etc with the exception of coolant

Aren't they all a form of oil that isn't compressible? Is it for temperature reasons? Seals? Why couldn't the same seal material be used for different systems if that's one of the reasons


1 Answer 1


The fluids are all designed for different conditions, more viscous so they "stick" longer, working temperature ranges, behaviour under pressure ie in the engine bearings etc.

Some of the early cars had hydraulic brakes - with water as the working fluid but in winter it was replaced with methylated spirits as it did not freeze... but it did evaporate rapidly. Water had the problem that it boils if the brakes get too hot so became the challenge of designing a fluid for the brakes that works in the conditions.

The brake fluid we have now is dimensionally stable over the working temperature range and, as a bonus it is an excellent paint remover...

  • Couldn't the systems be designed to use the same fluid? For instance why do floor jacks use a special fluid and not say just ATF or brake fluid. I read it has to do with the seals but why are the seals made differently?
    – user140123
    Aug 20, 2017 at 15:37
  • Seals are designed for the conditions - a floor jack does not achieve the same working temperature as a seal in a disc brake caliper ...
    – Solar Mike
    Aug 20, 2017 at 16:18
  • So basically the seals are made cheaper in something like a floor jack and thus a cheaper fluid? Though sometimes brake fluid is more expensive no? @SolarMike
    – user140123
    Aug 20, 2017 at 16:24
  • Down to the conditions that have to be satisfied - airplanes need a hydraulic fluid that works over a very wide temperature range etc.
    – Solar Mike
    Aug 20, 2017 at 16:26
  • So are they interchangeable at all? Say something made for more extreme conditions in something less like a floor jack using brake fluid or regular engine oil? @Solar Mike
    – user140123
    Aug 20, 2017 at 17:18

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