Fuel flow question 77 Chev C-10

I installed a see-through fuel filter on my 77 C-10. I noticed that at idle, the fuel seems to build up and then drain out and it looks as if there is nothing coming into the filter; then after a few seconds
there is a surge and it fills about half-way and then drains out again and it looks like nothing is coming in the filter. It just keeps cycling like that.
Is that a normal flow at idle? If not could the fuel pump be defective?
I've got dual tanks and I'm thinking maybe the pump is somehow sucking air along with gas.
Any ideas.
The truck idles, but sometimes seems on the virge of cutting out. The normal idle is supposed to be 500 RPM, for a California 350 smogger, but it seems low.
Moves OK at speed.

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