When filling from dry tanks I usually open the pressure release on the water heater. As the tanks fill eventually the water heater fills and water will spit out. Then I close the pressure release and continue filling the tanks. Once they are full I pull the kitchen drawers out and make sure the water filters have water in them. If not I remove them and fill them up, sometimes I change the filter cartridge also. Tighten up the water filters and then open the bathroom faucets and kitchen faucets and turn the pump on. Generally it primes somewhat quickly. Do remember a few times I was wondering if it ever was going to though.