FWIW, I always just pour 1 Gal of RV antifreeze into my freshwater tank and it let it pull from there. That way, I have my tank and its hoses covered. I DO make sure it's drained as much as possible. I always "blow" out my lines as well. I have a regulating compressor and set it on 40psi. Water heater drained? I "back" the antifreeze from my hot water faucet back into the water heater. Floyd showed me that trick and it works well.
I ALSO agree with one other poster though. I ran into the SAME issue not too long ago thinking my pump had gone bad! The reason it will run continuously, the "pressure switch" doesnt activate because there's no pressure on the line obviously. I was running off my 12v battery
which is old and tired. I plugged into shore power and away she went. That was probably a "priming" issue since the pump wasnt running max speed. I looked and I believe my battery
was at apx 12.2v or so. On the converter, it's apx 13.6.....AND the "amps" are there