I am going to make a couple of educated guesses here. Please let me know if I am wrong on either of them.
The first is that you did actually put the pump back in place and try to run it again after taking it apart. Therefore, even though you flushed the system and checked the pump outflow, it is still not working.
The second is that this is not a new boat
or a new bilge pump. It is really just a guess there, but I would assume if either of those were true, you would have said so.
However, I am going to first ask when the last time you are absolutely certain that the bilge pump was pumping water over the side? Have you ever confirmed that it did that?
I am going to say that your fundamental problem is that your pump is experiencing too much head
loss. That is generally the only problem that would give the symptoms you are describing. A centrifugal pump, like the Rule
you described, will pump fine if there is no pressure for it to pump against. But, if there is any resistance to flow, the pump may not be able to overcome it. This can happen with something as simple as having to pump the flow up too high a distance. A Rule 1500, for example, will pump 1500 gallons with no hose attached to it, but force it to pump up 6 feet from the bilge to the thru hull
and output drops by more than half.
Add an obstruction to the line that increases resistance but doesn't fully block the line and it drops even more.
Now add in a little corrosion
on the wiring
so that the pump isn't getting a full 12 volts of power, but is only getting 10 volts, for example, and the pump which was going great guns
with no hose connected (or a short hose only a foot or so up) will be unable to overcome the forces working against it.
Unfortunately, there is no easy fix for this. You can replace the wiring
and clean all of the connections, replace the output hose with all new line and increase the size of the pump to make up for the length of hose that the pump has to work
against. Any of these might fix the problem, all of them probably will.