I tried one more thing to test if it is somehow the solar system. With the pump off, I put the solar valve in the off position and closed the ball valve that I have leading to the solar intake. Then I primed the pump and system AGAIN. Then I turned the pump off and left it this way all night. In the morning, the pump was primed beautifully. As soon as I turned the ball valve open in the morning (pump still off), the pump lost prime as follows:
-SWG drains towards pool
-bubble appears in body of check valve at filter output
-bubbles continue to gurgle into the SWG from heater side a few per minute
-bubble grows in body of check valve at filter output
-check valve fails eventually and bubbles start to enter the filter a few per minute.

Where is the water going before the check valve at the filter fails? Maybe it goes back up the intake of the solar? That doesn't make sense! The solar valve is still closed... The swg is clear so I can see at this point it is empty (emptied as soon as I opened the ball valve) and no water is flowing through it.

Is the filter check valve failing earlier than I think and letting water drain back into the pool? I opened it to service it when all of this started happening and everything looked okay.

Help!!!