The valve that Danny is referring to is the valve that closes off water to the tank when winterizing. If it’s in the closed position, it can do what you described. Another thing to check is the end of the winterizing hose that comes into the wet bay. If the cap,is loose or if the seal is not good, that hose can suck air and cause the pump to lose prime partially.
The valve is located about 6” away from the pump. Just follow the pipe out from the pump and you’ll find it. You can reach it through the access door behind the wet bay.
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