Ok so how exactly do you gravity bleed?? Forgive me I am a body man. Yes I think the bowls got low enough they could have sucked air. Damn I thought I had gotten the air out. What you are saying makes sense though....
Gravity bleeding is simple. Just open up the bleeder, let the fluid flow out(in a tube into a jar if you want it to be neat). If at first you get sputters, thats air. It should eventually flow continously, then just close it and go to the next wheel.