In checking out past forum discussions I ran across references to bilge pumps creating an air bubble at the impellor intake and thus failing to pump liquid even when fully immersed. This would be analogous to "vapor lock" in a fuel pump. A suggested solution was to vent the intake area. I have another suggestion: tilt the pump so that any air bubble would be spilled around the side of the pump, perhaps even to the point of laying the pump on its side. I'm not familiar with larger units, but it seems to me the smaller ones would operate fine regardless of their attitude, AS LONG AS THE IMPELLOR IS COMPLETELY UNDER WATER. This would mean the switch would have to be above the highest point on the pump intake. The pump would leave a little more water in the bilge than it did before, but that would be a price I'd happily pay to know it couldn't vapor lock. There have been many reports of pump failure for "unknown" reasons - I wonder how many of them might have been ultimate burn out due to prolonged running in a "vapor lock" state.