When I don't drive my truck for a couple weeks or even an hour it is difficult to start due to the fuel filter bowl being half empty. I noticed today that after shutting off the motor I could see a small bubble appear and swirl around the top of the glass bowl and disappear followed by another. I think it was air. After about 30 minutes you could see the gas level in the bowl had dropped. I can't find any leaks or smell gas. Where did the bubble come from and where is it going? The gas tank is vented. I used vice grips to pinch the rubber gas line closed and the the appearance of the bubbles slowed significantly. If air is being drawn in wouldn't there be a leak somewhere ?

1956 3100 three speed 6 cyl. Stock with a few upgrades.