Originally Posted by layback40

Try clamping the fuel supply line before the primary filter one night and see if that stops it. If it does you can gradually move further up the system until you find it. I would have thought the one way valves in the lift pump would stop drain back. maybe thats the problem. When you changed the primary filter, maybe a little bit of crud got in. I have seen a hair sit across those valves and allow drain back.
You wont see air bubbles if the air leak is at the top of the secondary fuel filter. You could try putting a bead of RTV around the edge of the center bolt on the secondary one night & see if that fixes it.