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?