I had a Zodiac Pro before the Osprey, and I also had the same thing happen - I think that the valves are the same though maybe not?- Inside the cylindrical sort of thing there's a sort of "sticky out little pole bit"
if you twist that and make sure its pulled out as far as it will go it should reduce leakage a little, though I think the dust cap is supposed to be the final air tight seal. My tiny zodiac tender requires the dust cap screwed on fully to complete the air lock so I think its a common design feature.
Unless the tubes deflate significantly, I think there is nothing to worry aboput#!!!