There are not too many options on the low pressure side. I would be trying to isolate the problem further. My initial thoughts:
1. Check all the valves. Winterization and tank fill valves (the one by the pump) not fully closed could let air in. Mine has 2 valves to fill the tank from the winterization tube. If they are not both completely turned, I get nota.
2. Check if the pump strainer is tight.
3. Try using the winterization tube with plain water in a bucket if your system will do that. I think that would rule out the tank itself.
4. Try filling the tank with the winterization tube, if you have that option.
Added picture for visual of possible problem areas.