bjcsc, to apply the gas law to a system comprised of a liquid and gas at equilibrium is just incorrect. Also, brake fluid is formulated with a very high boiling point, very low viscosity, and very low vapor pressure. It is specifically designed to NOT do exactly what you claim, and outside of steep mountain descents or road racing, any shift in level at the master cylinder should be unrelated to this.
You claim the phenomenon the original poster reported is possibly caused by expanding brake fluid in a normally functioning brake system. If you still want to hold by that opinion, you are welcome to it. I say that is not the case.
Caps lock on is shouting, occasional capitalization is emphasis only in text messaging. But, I can't alter how you elect to interpret it.