Those units with have the valve panel I have done many this season (commercially), and yes the pump has to be primed before it will suck in winterizing condition because of the valve in the city inlet. All I do because I have todo so many have an outside pump with battery and pump through the city outlet and it will also go through the pump (on or off makes no different). This way it only takes me 15 minutes and don't have to prime the onboard pump. It is handy to have an spare pump anyway in case of an emergency so use it also for winterizing and with alligator clips you can connect to your RV battery. Membrane pumps are self priming, but when its pumping air (no liquid in the chamber) it has a hard time sucking the anti-freeze through the valve and also from a lower position. Hope this is understandable.
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