After seeing how long it takes for a 3" 16,000 gph trash pump to prime itself trying to suck water up 8', I think you'll have better luck if you give it all the help priming that you can. I had to keep the clean-out on top of the pump cracked open so the air would escape and it still took 5 min or so to get it primed by itself. There was water in the discharge hose, but the pump would keep sucking air that was in the suction hose. I'd keep the cap cracked so the air could get out, and step on the discharge hose get more water in the pump.
