I tried that trick once to get air bubbles out of Dot 5 and just about killed myself. The Dot 5 gives off some powerful fumes when it is hot and I haven't to get a chest full of the fumes. Not a good deal at all.
These days I stick with a high quality non Dot 5 brake fluid and change it on a regular basis. I find that if I flush the brakes every couple of years then there isn't any rust and pedal stays nice and firm.
I don't recall fumes being getting too strong. Then again, maybe my memory is fuzzy because of the fumes. But I heated up a pan of water and sat container in the water. Never got above boiling point of water, so maybe that wasn't warm enough.
