RE: New ballast system
Well, the pump has some sealing property, it leaks some drops but under high pressure (not this case); but the bag doesn't fully recover its original shape, it remains somehow shrinked, until the tube gets into contact with the air, then as air enters the bag it recovers its shape; the vacuum exists only while pumping, when I stop the pump, there's no more vacuum inside the bag; as for the WTC, the vacuum is very weak.
When using a pump, you will need a valve if you must keep air under high pressure (after gaving pumped water into the tight tank, but that's not the case.
And then the sub is on the surface, the pump seal is enough to prevent water entering the bag.
Please see the vis I just posted.
Regards