steel brake lines

I want to re-place the rubber brake hoses---what is the best way to separate the steel brakes from the rubber hose fitting/ I know heat but I am afraid of putting a hole in the metal brake lines years of corrision have really locked them up

If your steel lines or fittings are rusted or corroded or in bad condition, you need to replace them as well.

Ditto! Rusted steel lines don't bend like new lines. If you can't get the nut free with a reasonable effort, the line should go the same way as the hose. Even if it's trustworthy now in it's current state, after the abuse of separating it it will no longer be trustworthy. Brake lines are cheap anyhow.