A buddy's truck had a bad solenoid, and tapping it got him home that night.

Had the same headache with a 4 inch on the American River last summer, losing prime every half hour and me cussing up a storm. Turned out it was a hairline crack right at the fitting, not even a pinhole, so the whole hose was done. What finally worked was pulling the suction line and filling it with water while the other end was capped, then watching for drips on the ground, found it in about two minutes. I've also heard of guys pressurizing the line real low with air and spraying soapy water on it, bubbles show up quick. Your capped pipe trick is smart though, keeping that water column in there beats sucking air all day.

LEDs really are the killer here. The old sodium streetlights put out light at very specific wavelengths, so a filter could block those narrow bands and still let through most of what you want from the stars. LEDs blast light across the whole spectrum, so there's no clean gap to cut out anymore. A filter built for the old lights just ends up cutting your nebulae too, which is exactly the teal gray mess you're describing. You'd almost need a different filter for every neighborhood, and even then it's a losing fight. That drive out past Apache Junction is doing more than any piece of glass ever will.