New brake line leaking. on 1999 GMC Sierra 2500

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I replaced all the brake lines with Cunifer then bled them at the wheels. The one that goes from the control box (under cab) to rear wheels is leaking. It is screwed in fully and tight but when pressure is applied fluid comes out around the line/fitting. I flared them all the same but none of the others leak. I tried to cut and re-flare it, same problem. Does this one use a different flare? Could it be a problem with the control box? Not sure what to do. Help!
(1) Answer
(3) Comments
What do you mean by a "derby?" You mean like dirt or debris that needs to be cleaned? Use brake parts cleaner? Thanks for your help.
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