I believe (I could be wrong), that the ballast tanks were vented to the outside world from the bottom. The air was pumped in from the top, so you have to completely blow all the water out before any air escaped.
From what i seem to get from my readinsg, is that the American subs used big, honkin' blowers for the ballast tanks. You'd likly give yourself away with those monster things before the bubbles gave you away. All this is just my impression though, it may be totally off...maybe a more informed poster can clarify.
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