Tom, I wanted to say it was not enough vent as well, but when I looked closer, the other tanks appear empty with the last tank full. Not enough vent would not have allowed fuel flow at some point, unless the suction of the last tank was such that it drew fuel back through the system after the pump was turned off, but that would still not explain the draining I'm seeing when he lifted the tank.

This is a little perplexing. Can't wait to hear the final results