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When reading the excellent answer to the question Is failing to change brake fluid a life safety issue? I started to think about the physics of brake fluid heating up. The brake lines are fairly long and thin. Brake fluid can heat up from radiant heat in the engine bay and from brake heat. But how hot can it get?

The radiant heat in the engine bay should probably limit brake fluid heat to about 50-70 degrees Celsius for the front brakes (scientific wild ass guess, I should probably go and measure my engine bay using an infrared thermometer after a long drive). The engine itself is 90 degrees Celsius typically, but it is cooled and the brake lines are quite far away from the engine, so I don't think engine heat alone can heat up the brake fluid to 90 degrees Celsius.

The usual argument for changing the brake fluid is that the water in it can evaporate, leading me to believe that brake fluid can heat up to 100 degrees Celsius. Is this correct? If so, how can this happen if the steady-state equilibrium temperature from engine heat is 50-70 degrees Celsius and the brake lines are very thin and long? It isn't a circulating system like the cooling system!

Or is it actually the case that evaporating absorbed water is happening only very near the brakes, and the fluid temperature in the long lines is mostly much below 100 degrees Celsius?

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  • I'm sure you realize that at the caliper the brake piston is in contact with the pads and also in contact with the fluid. They are metal, so that transfer of heat is pretty direct. At the wheels is certainly the biggest concern.
    – JPhi1618
    Apr 4, 2018 at 19:37
  • You are correct that the absorbed water is the issue. The water vaporizes into a gas. Gasses are compressible. The result is a lack of fluid pressure at the caliper and poor braking performance.
    – mikes
    Apr 4, 2018 at 20:16

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Short answer: It gets REALLY hot - like, way over 100C (decimal). And, as @SolarMike's answer states, the water that the fluid absorbs from the atmosphere likes to vaporize under those conditions. Brake systems are technically closed, sealed systems, but no seal is perfect.

Put another way:

Heat in the engine bay is a non-issue for brake fluid. All of the drama happens at the caliper. Or, more specifically, at the pad/rotor interface.

Your brake pads press against the rotor - or shoes against the drum, if you forget to take off the e-brake and do a burnout - and, through the magic of kinetic friction, motion becomes heat.

Rotors serve two purposes: the most obvious is to provide a friction surface for the pads. The second, perhaps not-so-obvious is as a heat sink. All that heat that's generated while braking has to go somewhere, and we'd prefer for it not to go into the brake fluid. (Heat destroys everything. Just ask the Universe.) The pads absorb some of it, and conduct a lot into the caliper. (Which happily passes it on to the fluid through the caliper pistons.) The rest goes into the rotor and the air (yay, radiation).

On the track, I bleed the brakes at least twice per day. Every time, my nice, brand-new fluid is baked to the point where it has turned black. (This is after about an hour of high-speed driving and hard braking.)

Back when I had my Corvette, one of the popular mods among the road-racing crowd was to replace the aluminum brake caliper pistons with stainless steel. Steel absorbs more heat than aluminum, but, more importantly (but related), it doesn't conduct heat as well as AL. This kept more heat in the rotors, which behave much better when overheated than brake fluid does.

If you Google a bit, you can find many pictures of cars on race tracks with glowing brake rotors. The heat produced is insane. (This also happens if you happen to be driving around with brake "pads" that have no "pad" left.)

Race car with brakes glowing cause they're wicked hot

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    The wikipedia article on brake fluid is quite illuminating - it has a table showing the difference in boiling points between dry (pure) brake fluids and wet (3.7% water) - common DOT4 fluid boils at 230C if pure but just 155C when diluted. en.wikipedia.org/wiki/Brake_fluid
    – Matt Bowyer
    Apr 4, 2018 at 22:41
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    I regret that I have but one upvote to give
    – Leliel
    Apr 5, 2018 at 6:14
  • @Leliel And, finally, my public-school history education comes in handy. :)
    – 3Dave
    Apr 20, 2018 at 0:15
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If there is a small amount of water in the brake fluid in the caliper and the brakes are used severely for an extended period the heating can cause the water to change to vapour.

The issue is that brake fluid (except dot5) is hygroscopic - that is it absorbs moisture. This happens slowly over time as the brake fluid reservoir has a small vent in it to allow air in. The system has to breathe as when the pads wear fluid is drawn in to accomodate the wear.

What is not realised is that 1 litre of water expands to 1700 litres of vapour (roughly without exact temperatures and pressure - not got my steam tables to hand...) - if you have left a pan of water on the stove and come back to find the kitchen full of vapour...

So, a small amount of water expanding in the brake caliper can easily cause the brakes to bind or not operate correctly.

That is why changing the fluid at the recommended intervals is recommended.

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    Unless it is dot5 fluid, which is not hydroscopic.
    – Moab
    Apr 4, 2018 at 22:45
  • @Moab did you mean hygroscopic?
    – Solar Mike
    Apr 5, 2018 at 5:05
  • D'oh, yes!!!!!!
    – Moab
    Apr 5, 2018 at 22:30

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