PARTSGUY - NEED HELP PLEASE

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Not understanding the Prime every time statement. Sure that isn't just the first time?
There is a one-way check valve between the unit and the reservoir that prevents fluid from going back into the reservoir, so how is the unit going dry? The unit, whether on or off, just allows the fluid to be pushed through from the pump.
I would understand if you run out of fluid it would require priming again.