I believe there's supposed to be a small spring at the bottom of the glass reservoir that holds the filter against the top housing. At least that's the way the ones I used 50-something years ago worked. The "air" you're seeing after shut down is probably vaporized fuel created by the under hood temperature. As soon as the fuel pump kicks in, the pressure and the gas flow through the filter prevents the evaporation from happening while the engine is running.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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