When refurbishing a cars fuel system and not replacing the hard lines I always flush them with solvent. I rig up a rubber suction hose to the hard line and a second line that "Y's" off of it and used a compressed air nozzle to suck the solvent through the line. I use acetone and run it through the lines until it runs clear. When done blow through the lines with compressed air and done. I line acetone because it dissolves most anything, It evaporates quickly, and doesn't leave a residue.