Towing: Which line to "pinch off" when towing?

[Sportster-2001-951C-Stock]

That corrugated tube from the bilge pump should have a constant upward slope rising from the pump to the outlet through-hull fitting. If there's a low spot that can cause an airlock. Say for instance someone cut a ziptie while doing some work, this might alter the slope of the corrugated tube.

But it's pretty common for an air bubble to become trapped in the bilge pump but when water level continues increasing the air bubble will eventually clear and pumping will begin.

 

[lineofsight]

Pump or hose is plugged. It can also be a sheared shaft at the impeller of the pump. But not likely.

Will add this to Spring checklist - possibility had not entered thinking and as I leave the boat on a dock overnight for a week at a time at least twice a year...