How co-incidental is this. I spent this week resurrecting the 10-15 year dormant fuel system on a 78. Every section of the hardlines, including the fuel rail, was plugged including the fuel pump. A lot of the old fuel sludge blew out with compressed air, but no such luck on the fuel tank itself. The tank had 40 litres of the smelliest grossest sludgy crap I've ever dealt with.
I tried the poke it with a wire method with no luck, then I resorted to petro-chemicals (I used Laquer thinner) to melt the stubborn blockage in the return line. I hooked up a 3-4 foot chuck of 5/16 rubber fuel line to the return tube, and hung it vertically up beside the rear fender to get it higher than the tank. I then filled the hose with laquer thinner and gave it a couple of hours to work on the crud it was now in contact with.
I tipped the hose down into a low pan and out poured first clear laquer thinner then browner and browner then dark rust/crud/sludge. I repeated this process one more time, got more crap out, then applied compressed air the line and PHOOOMMM blew out the last of the crap. The old Z now purrs like a kitten. Required a new fuel pump too.