Geez, 68, you have got some good info on what is happening and have done all the right things to try to fix it. I would have suspected pulling a vacuum in tank if it only happens after a long run, but you checked for that by pulling the gas cap.
Or check valves in the pump but you tried new pump.
Or dirty sock in tank, but, if I understand you correctly, you connected pump suction to a gas can and still had the problem.
You have tricky one!
Does it only happen after a long run? Is it a Chevy or a Pontiac engine? What carb?
I suggest you post in "Drivetrain - Street" since the guys who read this forum are too obsessed with getting their Pontiacs to run cooler to give this the attention it deserves
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