Thanks for the tips. I inflated the keel tube hard, sprayed some soapy water around and found a big leak right at the A7 valve assembly. It was leaking between the base of the outer part of the valve and the disk that clamps down on the fabric. When I took it apart I also found the thin rubber gasket between the disk and the outer assembly was missing. I borrowed one from my Redcrest and after tightening everything up it looks like the leak has subsided. Still a few small bubbles, but much less than before. By the way, it wasn't loose when I first took it off, something must have been out of alignment to cause the leak.
How much torque can you put on these valves? I'm concerned about over-tightening it and cracking the valve. I've tightened it quite a bit more than the tube valves, but that was the only way to stop the leak.