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Most cars use DOT brake fluid, which is hygroscopic. By absorbing moisture, it needs to be periodically changed, typically every 2 years. Thus, for example for a 20 year service life the brake fluid needs to be changed 9 times in its lifetime in addition to the initial brake system filling. On an internal combustion engine car this can be done when an oil change is performed, but electric cars lack oil changes and thus the brake fluid change is the only reason why you would like to bring the electric car in for service.

This requirement to change the brake fluid adds major maintenance costs to the owner of a car. Considering that manufacturing typically becomes less expensive over time but labor becomes more expensive over time, the need to change brake fluid can be a major obstacle for many to owning a car.

Bicycle component manufacturers have chosen a different kind of brake fluid. All Shimano hydraulic disc brakes use mineral oil as the brake fluid. It generally doesn't deteriorate. It is not hygroscopic so the moisture absorbing problem is not there. A once properly bled braking system typically lasts easily for 10 years or more with no bleeding or fluid changes required, provided that the bicycle is not routinely stored upside down, a problem which might affect some bicycles but no cars as nobody would ever park a car upside down.

Thus, there seems to be many advantages to using mineral oil as the brake fluid instead of DOT fluid. Car manufacturers however have not switched to mineral oil. Why is this the case? Is there some advantage of DOT brake fluid that I'm not seeing?

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  • The temperature range - it does not freeze and rarely boils. Early systems used water and then added methylated spirits with all the disadvantages...
    – Solar Mike
    May 30 at 9:22
  • Mineral oil decomposes the rubber seals. Neoprene has more resistance but lacks the low-temperature flexibility of rubber. May 30 at 12:14
  • The US Department of Transportation produced a DOT 5 specification which was silicone based and hydrophobic. The only "minor" problem was that it didn't work with anti-lock braking systems because it could absorb air when the ABS system operated.
    – alephzero
    May 30 at 12:43
  • A factor against change is that most different fluid formulations that have been tried are incompatible with each other. You don't want to have Joe Klutz regularly topping up his slightly leaky brakes with the wrong fluid type, resulting in total brake failure the next time he tries to do an emergency stop.
    – alephzero
    May 30 at 12:48
  • It is the air than can be dissolved in water that makes water unsuitable. It becomes compressible so the braking effort is diverted from actuating the brake to compressing the brake fluid. May 30 at 13:11
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The Bosch Automotive Handbook confirms that mineral-oil-based fluids are not hygroscopic:

Mineral-oil fluids (ISO 7308)

The great advantage of mineral-oil-based fluids is the fact that they are not hygroscopic, so the boiling point does not drop due to moisture absorption.

It also makes clear that mineral-oil fluids are incompatible with systems designed for with glycol-ether fluids (like DOT3, DOT4, DOT4+, DOT5.1):

Mineral-oil fluids should never be added to brake systems which are designed for glycol fluids (or vice versa), as this would destroy the elastomers.

But perhaps this is the real reason: since the moisture doesn't get absorbed by mineral-oil-based fluids it'll just sit there inside the brake lines, freezing and evaporating as dictated by ambient conditions. That's absolutely unacceptable for hydraulic systems:

A critical factor with brake fluids based on silicone or mineral oils is the absorption of free water in a fluid state, as the water forms vapor bubbles when it heats up to more than 100 deg C and freezes when it cools to less than 0 deg C.

I imagine that cyclists don't have to worry about boiling the moisture collected in their brake systems to the same extent that automobiles do.

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  • Both my sons do MTB and yes they have to make sure the brake fluid is correct otherwise they end up with no brakes as they get very hot... But if you mean going shopping on a butcher's bike...
    – Solar Mike
    May 31 at 12:12

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