When disconnecting a brake hose I twisted off a steel brake line between the driver side caliper hose connection and the combination valve in my 1992 chevy g20 van. A friend told me that when you replace one brake line you have to replace them all because the pressure changes related to the new line will likely cause the other lines to burst. Is this true? I figured there is some sense that one rusted brake line probably means the other ones are rusted as well, but if the other lines are fine is there really any other reason they should be replaced?
Whatever your friend is, he’s certainly no mechanic and knows little of physics. One assumes the line you broke would not have failed any time soon had you not twisted it off, so the problem was not rust, it was the way you removed the line. You need to hold the nut and turn the fitting. As for the pressure changes, they’re non-existant.
Change the broken line being careful how you do it, bleed the brakes, and take no further mechanical advice from your friend.
You said it all. What the friend might have overheard is that you should replace brake LININGS in sets (right and left) to retain even braking. What used to be called folklore is now better classified as “Junk Science”. Your friend may be watching too many TV talk shows or read tabloid newspapers.