Thanks to all who responded both on the LML and privately.  In the end I had an issue with incorrect pinnout on a DSUB for the right wing input.  When I discovered  that problem (after two days of troubleshooting and testing wires) and corrected the pinnout and drawings, the problem went away.  And for the record the system easily recognizes one gallon of fuel ( I have the Garmin G3X).   I have an old kit so the probes are the 8 foot probes originating at the wing root.   I am very pleased with the probe overall.  I added a gallon at a time to the wings and got  a good detection of each gallon up until about 39 gallons in my 40 gallon wings.   I have heard stories of high quantities of unusable fuel in these birds.   Granted this is not a completely valid test because it is not simulating the backpressure of the engine, but the pump loss of flow coincided with the fuel level reading zero gallons.    I would be interested if others had experienced similar results.   I am curious as to what the real world experience has been on unusable fuel and how you tested it. 

 

Thanks again!   Dennis