Problem with lawnmower

My lawnmower is a basic model (cost about $139.00). It has worked ok for 3 years until recently. It starts up ok and runs for a while, but the bulb you use to prime the engine prior to starting it gradually gets sucked back in, into the "pushed in" position and then eventually it will stall. I have to open the gas tank, let it cool down a bit, and then sort of pry the bulb back into the "out" position. Then I can prime it and it will start again and run for a while before it does it again. It seems like the hotter the engine is, the faster the bulb gets sucked in and it stalls. Obviously it seems like an air intake problem but how can I fix it? New cap for gas tank maybe? Thanks for suggestions.
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Might be a clogged gas cap vent. Run it with the gas cap real loose, probably dirt in cap.
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