And IMHO the camera shop owner is completely wrong! It does not make any difference if you pull from inside the housing (= apply a vacuum) or push from outside (= apply water pressure). So the idea that the o-rings are "sucked in" is nonsense. And nonsense especially if you think about the relatively small degree of vacuum you apply with your pump.
Bottom line: If you want to avoid a flooding don't take your housing under water :-)
Just my two Euro-Cents
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