If you have frozen rusted lines, then if you don't have to touch them, you shouldn't. If you do, then clean up the whole mess. Sometimes trying to just fix only one gets you started and then you get sucked in deeper and deeper. I will not re-use a steel line where the flats on the nut have stripped off trying to remove/install it to the point where you can only use vise-grips to tighten/loosen it. Then trying it not to pinch the line can be difficult. Time to replace the line with a new one...
If you try to just fix part of it, you usually end up getting into it deeper and deeper. If you have to go replace the steel lines, you may as well make sure that the rubber hoses are in good shape also.
