OK, short answers: No, there shouldn't be any air pockets, as the manifold fills up with water along with the tank, and there is ample space for air to escape through the intakes and powerhead. The powerhead is fully under water.
Here's a top view of what I've built:
The manifold intakes are in the main part of the tank. The powerhead is in a separate chamber, partitioned off with foam board which rises to within 1" of the top of the tank. The water in the main tank is filled to within about six inches of the top. I had hoped that the powerhead would fill the chamber with water, overflowing at the top back into the main tank, making a nifty looking waterfall in the process. The concept is fairly sound, I think, and the water did rise to within an inch of the top before it stopped. I think that the pump simply isn't powerful enough to overcome both the leakage and gravity. So, I drained it; once it's dry I'll seal better around the bottom. If that doesn't do it, I'll get a more powerful pump.
Probably should start a new thread about this, but hey, this IS part of what I did today, so I guess we're not too far off-topic.
