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.