I wouldn't replace the metal lines unless they obviously leak or are damaged. I am very glad I didn't have to replace mine. Would've been good experience though...

Use brake line wrenches of high quality so you don't damage your lines. I've just had to do this very job and had this exact same issue as you're having; good wrenches definitely help.

P.S. Check your parking brake shoes while yer in there.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)