Starting about 1966 or 67 Pontiac had pumps with the vapor return instead of the return off the filter. I suspect your "new" filter may be defective and have too large a hole into the return line. Take the filter off and see if you can get a look into the little 1/4 inch return nipple. If the hole at the top surface of the filter can is much more than what you could make with a needle it may be too large and is bleeding off the pump pressure when the return line is open. I just looked in a spare filter I have and I doubt the hole is 1/64 inch.
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