On the 1991 560SEL, I'm attempting to remove the OLD fuel supply line (line that comes out left side of fuel filter and connects to a metal line which sends fuel to the engine) and install a NEW fuel supply line. When trying to remove the OLD fuel supply line, I first used brake cleaner to clean up some of the old grease and grime off the nut. Next I sprayed the threads with PCB blaster to loosen things up.

I can get a 17mm angle-adjustable flare nut wrench on it but when I try to turn it, the metal line flexes and I'm afraid if I turn very hard I might break the metal fuel line it attaches to.

Question: Is there a method for securing/bracing the metal fuel line so that the 17mm nut can be turned? Thank you for help/input/assistance.

Note: I edited this post to clarify what I am attempting to do, take off the OLD and install the NEW fuel line.