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Jack, the Rule-Mate 1500 appears to be an automatic version of their #2 pump. Oddly not well documented on their web site. And even more oddly, Rule shows the pump as 4.8A@12V and 7A@13.6V, which sounds like they got the numbers reversed. (Motors draw MORE amperage as the voltage goes down, not less.)
So first, assuming it needs 7A at battery voltage, and the wire run is 30 feet from the battery, that would require a nominal 60-foot run adequately sized for 7A service. My wire gauge charts show 5A service requires 10AWG and 10A service requires 6AWG, so you might actually need 8AWG wire to supply the power that pump draws! And that's assuming all crimps are solid and clean as well.
First thing to check: With a voltmeter, when the pump is turned on and running (disconnect the discharge hose) do you still get 12V at the pump while the engine is not running? How does the voltage at the pump (or as close as you can get) compare to at the battery?
Second thing, the head (lift) rating for the pump. It seems to be rated at 1200GPH with a 3.3 FOOT lift, and drops to 800GPH with a 6.7 foot lift--and those are both with the full 1-1/8" hose. If you've got a four foot lift, or five foot lift, and you are reducing the hose size, I'd still expect it to pump 100-200GPH though the smaller hose. That it is not pumping AT ALL might indicate the impeller is slipping on the shaft, or that the motor (as above) isn't getting enough power to turn against the extra restriction.
But I wouldn't reduce the hose size at all, if you're going to do that you might as well just buy a pump that is half the price and designed to use that little hose! Use the 1500 as a damage control pump, and then add in a small--very small--pump using a very small hose as the "squeegee" pump, which sucks the very bottom of the bilge. Move the 1500 up an inch or so above the bottom, so it isn't used until and unless there's really a need to move water, and that way you don't have to worry about the "big" slug of water coming back down the hose when it cycles.
Yes, using two pumps really is the simplest way to accomplish this, without compromising the capacity of the big pump, and without leaving water in the bilge.