If your boat has the airator type pump then i can only say vent it vent it. I had all the same frustrations and the worst was i had a 400 that wouldnt even drain while full because a air pocket got stuck at the pump. I was literaly jumping up and down on the sac LOL. The biggest problem with air pumps is if you get any air in the sac and it makes it to the pump you get air locked.

Now if your running reversables, then i cant help you as much as i just installed mine and havnt hit the lake yet.