Brake fluid boiling point is a very good comparison for evaluating regular or polyglycolether based fluids, "glycol" for short, because it roughly represents the conditions under which glycol turns compressible. On the other hand, it is a very poor comparison for evaluating silicone based fluids because they turn compressible at a lower temperature than that at which they boil.
To add to the confusion, (presuming a properly bled system with negligible air bubbles), non-silicon fluids get compressible only after they boil (with absorbed moisture lowering this temperature), however the silicon fluid is four times more compressible at elevated temps (but before boiling point is reached). This is also not helped by the susceptibility to micro-aeration. This is the crux of the problem when discussing the fluid options.
Glycol based brake fluid is hygroscopic, i.e., it absorbs water from the surrounding atmosphere, and it starts adsorbing water as soon as you put it into your car's brake system. The brake fluid reservoirs on top of the master cylinder are vented to the atmosphere. Moisture can also enter the brake system through the rubber seals and hoses through a 'diffusion' process
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