The modern replacement gas caps have a valve that seems to stick often causing a vacuum condition in the tank.

See if you can breathe through your fuel filter. If you can't that's the culprit. Cut your filter open and see if it's clogged full of crud.

1950 Chevrolet 3100 (Ol' Roy)

1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)