There must be a suction tube the fitting screws into or it wouldn't work at all without the tank being right full.
Given how horrible this model company's machining tolerances are with cyls & valves leaking from new, chances are the tolerances in the pump aren't perfect either, and that's why it won't prime on its own & won't hold prime during periods not in use.
For sealant, I've been using the same liquid putty sealant that I use with plastic water fittings. I'm sure a blue or red locktite would suffice too.