Knot,
You must be taller, thinner, and much more nimble and I am. The primary sensor I checked through my pre-departure check is the one most often used the aft sensor. On my 2010 R 25 The sensor is located under the propeller shaft, under the exhaust hose, under most everything else on the boat-exactly where you would expect. To reach the sensor I must lie on the cockpit soul, Reach down under the propeller and reach under the sensor the lowest place where the contacts you described are located.
Prior to replacing the sensor I removed and cleaned it exactly as you described. The normal very small quantity of diesel, oil, soap and other cleaning solutions creates the crud deposit you describe causing the sensor to either turn on the bilge pump switch when there is a little water present or not turn on the switch when the bilge pump should be pumping.
I cannot reach the forward sensor for routine testing. I cannot reach the sensor for the dash console indicator because I cannot find it just by tracing wires.
Everyone runs their own boat and makes their own decisions. My decision for my boat is to install one of the reliable Rule bilge pump sensors that I could test easily with a bent coat hanger without becoming a pretzel to perform the test.
I don't mean to give the impression this is the way to do it. That would be presumptuous. I only want to share the idea that the combination of the scupper screens and the electronic sensors cost me some significant repairs when ordinary cleaning project flooded my bilge because neither the forward sensor, the aft sensor nor The mystery emergency sensor for the console flooding Alarm detected the water and turned on their respective pumps.
In addition to replacing the sensors, except for the one I can't find, I added three standalone water detectors available from Amazon for just two or three dollars each. They are not powered by the ships electricity and between the three of them, I feel at least one will sound if I have sensor or bilge pump failure again.
I appreciate the discussion and hope others benefit by having the incentive to test their bilge pumps. At very least they could put a few gallons of fresh water in the bilge and see if it gets pumped out.
Stuart Bell
Ranger 25: Shearwater
(561) 352-1796