Shurflo 4008

Pump is self priming however on the initial use after winterizing it can take up to 5 minutes to pressurize the system.

Three issues that can cause issues...

1: If you opened the inlet strainer housing and drained it and did not get it properly closed 9Rare but it can happen)

2: Closed tank outlet valve or open drain on some systems (Again not all that likely)

3: (Suspect #1) Open low point drain valve

All I did was drain everything from all the places I could, and blow air thru until it quit spitting. Will pay better attention next time.

DO you have a wet bay?
A water panel?

Fill valve?

Winterizing valve?

Could be fill valve still open allowing water to round robin to tank
Could be winterizing valve allowing air to get sucked in on suction side of pump
Could be cracked suction strainer...pump casing
Could be pump itself

Shurflo 4008 pump has an internal bypass valve.
Bypass starts opening at 40# and is fully open at 65#
But if stuck open pump can't build pressure or pump water to system

Sounds like one of the winterization valves may be allowing the pump to suck air in the line. If you have a single lever Anderson valve you may start there. If you have the traditional open ended hose at the pump for putting in the antifreeze, you might check that the valve to the fresh tank is fully open, and the valve to the suction hose is completely closed if you haven't already done so.