needle tank vent, open or closed during operation?

[Lil Giants]

That's what the problem is, you get an air bubble in the sump line & that gear pump isn't self priming. Maybe closing the vent when not in use will prevent that, but it should be open when it's operating.

Threading a fitting into a plastic fuel tank, carefully drill the correct under sized hole, manually thread fitting into tank, unthread it, then add sealant to fitting threads & screw it back, not over tightening fitting pulling out threads. Let it dry 24hr. I've done a half dozen this way & have never had one leak.