as majicwrench wrote, the fuel can't siphon from the float bowl back through the needle and seat into the fuel line. Adding a check valve won't have any effect on the float bowl draining. Since the base and center section are all one piece, you either have a leaking accelerator pump or a cracked carburetor. Just because your carb is "rebuilt" doesn't mean much. I've seen accelerator pumps fail within a few days of exposure to our reformulated fuel, and more commonly, warped pump covers that allow some seepage.
Whichever Fiat supply houses gave you that info should quit having sales people give advice, they're obviously more versed in sitting in front of a computer than actually fixing a car
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