The master cylinder should not heat up to any significant degree as the
result of hard braking, although the wheel cylinders can certainly heat up
(and overheat) as a result of the heat generated from braking friction.
However, if the master cylinder is heated from its proximity to the exhaust
(or any other source), it will, as you suspect, accelerate the evaporation
to SOME extent, although I doubt that it would cause a rapid loss of fluid,
even in a non-sealed system. Part of this depends, of course, on just HOW
hot it gets. If it really is getting too hot, the exhaust might need more
heat shielding, although even a single-layer heat shield can greatly reduce
Fluid losses always result from some combination of:
- Normal seepage around seals.
- Unwanted seepage and leaks from bad joints, worn or damaged seals,
corrosion, or other physical damage.
- Perhaps the odd molecule or two that manages to penetrate a membrane.
- Midnight brake fluid thieves.
The better the condition of the system, the less seepage there is. Unsealed
(vented) systems will have somewhat higher evaporation losses than sealed
systems. There will always be a tiny bit of seepage and evaporation at the
wheel cylinders, even with just-broken-in seals and factory-perfect cylinder
(Salt Lake City, Utah)
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