I have had problems with the Rule type bilge pumps becoming 'air bound'.
This seems to be the result of the swith being set to low and thus the pump
runs until the water lever too low on the impeller cage, sucking in air..
The next time the level raises there is air trapped inside the impellor cage
and the impellor has trouble getting an intial suction.

I have three bilge sumps and I have discovered this problem in two of them..
I am going to have to raise the float level to over come this 'air bound'
problem.


--
My opinion and experience. FWIW

Steve
s/v Good Intentions