Originally Posted by canuckdave
I checked the debris screen at the pump intake but it was clear and I can see air in the wet bay lines leading to the pump.
It sounds like the pump is sucking in air from somewhere upstream, and until that is resolved you will never get all of the air out of the downstream (pressurized) areas.
I've had similar symptoms to what you describe when the pump is sucking in air. What usually causes it for me is if the cap for the debris screen is even the slightest bit loose.
Another easy place to miss is if you have a winterizing inlet that allows the pump to suck in the pink stuff antifreeze. Make sure the valve to that line is completely closed. If it's open just a crack, that can also allow air into the pump intake.
On my rig, the line from the tank to the pump continues on to a valve and a lot point drain. I suppose it's possible that the drain valve could also be open a crack, allowing air into the line when the pump is drawing from the tank. I would think that would drip when the pump is off, and you would've noticed it, but I guess it's possible (but unlikely) that the opening in the valve is just big enough to let in air but not let out any significant water.
You are seeing air in the line from the tank. I know it's tough to follow those lines, but you will need to follow them upstream to see where the air is getting in. Hopefully it's something simple like the ideas above, and you won't have to resort to tracing the pipes.