I don't use a neptune pump, but same difference- you can put a tee at the highest location- preferably still above the sump. Then put a valve on the line to restrict the line-- it will lose little if any water, but be able to suck plenty of air. Or you can use a bit of tube and tie it in a knot to restrict it- I've been running mine that way for years with no problemsHow are you stopping the pump from siphoning all the water out of the sump when it shuts off?