I got a leak in my 85 2gm20f about Jan 1. the gasket went haywire. I did not have a gasket so found and made do with some gasket material. was too big, so no fluid would pump thru. Removed the gasket all together, and used some Tstat silicone tween the back and the main unit, and it works fine now. Unfortuneatly, had a small over heat issue last sunday in some 30-40 knot winds!
Otherwise, I am not sure what to say re losing the prime. As i was checking initially to see why no water would go thru. One of my tests, was to open the valve with the pump not connected, and water would flow up thru my hose. Next step was to see if any got to the inlet in the radiator part. Nothing. So that left the pump for me anyway, not working at all.
Not sure why, other than maybe something is floating around in the inlet that you can not see, ie a rock, small sea shell type creature etc
She drives me boat,
I drives me dinghy!