Mercedes does not use the standard double flare. They have a metric spec bulb type of flare that is not compatible with the normal doulbe flare. If you go to most parts stores they have brake lines with the fitting already made. They have US, British and european metric, maybe more.
Make sure to take an old line with you to visually match to type of flare.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags