I think the check valve helps to keep the pump nozzles primed with fuel.
Each carb bowl should have a ball, weight & clip, and each nozzle should have an o-ring at the base.
Also, the check valve may keep the accelerator pump circuit from flooding or something like that (
see quote):
By WG: The check valve is in each of the four bowls and keeps gas from just filling the accelerator pump circuit without any restraint. There is actually a valve in the bottom of the #2 bowl that you can't get to and often clogs with today's bad gas and then gas is forced into the other pump circuits (the small gas hose hung on the lateral gas barbs on the bowls) through the check valves. The check valves consist of a bb, a small brass "rod" and a wire clip to hold the latter two bits in place. The only way to buy replacements is to buy an accelerator pump kit which includes those items.
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