OK. Now I understand your goal. (You can call me slow, but you can't fault me for not being thorough...
)
One last test. With the sensors hooked up, the Reserator powered up and with coolant in the reservoir, is there a detectable DC potential (with respect to the
circuit Gnd,
not the chassis Gnd)) on either of the sensors? Or across the sensors? I would expect it to be <=1V, if present (anything higher would cause unacceptable electrolysis of the cooling fluid).
If there is a DC potential, I'm going to assume that the water level sensor is monitoring conductance.
I'm confident that the following trials will not damage your Reserator's electronics.
Do you have access to a local Radio Shack (or equivalent)? If so, purchase a 5 pack of 10M ohm 1/4 W resistors (RS Model: 271-1365, they don't appear to have 1/8W available although 1/8W would be adequate).
Now, with NO coolant in the reservoir, connect all of the resistors in
series (50M ohms total). Connect the two ends of the group to the sensor wires and observe the response of the Reserator when powered up.
Depending on the response, repeat the above after
removing one resistor at a time until only one is left and observe the responses.
Finally, if you still haven't gotten the response you want, start adding back the resistors,
one at a time,
but in parallel, thus further reducing the total resistance seen by the Reserator electronics.