5.5 HP B&S Sabre Engine Leaks Gas into Air Cleaner When Running

I have a 1996 John Deere 14PS push mower with a 5.5 HP B&S Sabre engine. Last weekend after I finshed mowing the lawn, I noticed gas drips on the deck under the air cleaner housing. When I opened the housing, the bottom part of the filter element was soaked with gas. This model has a float carb with no adjusting screws or choke and no throttle control, but it does have a primer bulb. I cleaned the deck up and no more gas leaked out while it wasn't running.
I took the carb apart, and it didn't have any dirt or sludge in the bowl, so I gave it a quick clean with mineral spirits (didn't have carb cleaner) and replaced the float valve and seat then reassembled it with new gaskets. I put in a new filter element and mowed half of my yard before I noticed that the gas was dripping from the air cleaner again.
Just an observation, but the little nib in the center of the primer bulb has a hole in it. I'm not sure whether this is normal or even if not whether it is relevant to the problem. I've had it suggested to me that perhaps the intake valve isn't seating properly, such that the airflow in the carb throat might momentarily reverse during the compression stroke. Any ideas?

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