I had the same problem with my '53 several years ago. Pulled the fuel line at the gas tank and the other end at the fuel pump. Shot some carb cleaner down the line and nothing came out the other end. Kept putting carb cleaner in one end and hung the line up so it could eventually drain through. After about a half hour I got about a foot length of black stuff that finally snaked its way out. I ended up draining the gas tank and then put fresh gas in. No problem after that. The carb cleaner worked real good though. That may be what your problem is if you have a buildup of garbage in the bottom of your tank.

1953 Chevy 3600 3/4-Ton