I would be worried about current flowing from the sensor(s) through the coolant to other parts of the system (notably the WB). Just because it is a longer route doesn't mean no current will flow through it. If you run a 1Ohm and a 2Ohm resistor in parallel you don't get 0 current through the 2Ohm, you just get 1/2 as much. And it wouldn't take very much current through your CPU to fry it. If only 1/10th of the voltage you have in your sensors leaks back through your WB thats still (according to your diagram) .6V through the CPU, which is a lot for a CPU - I wouldn't want an extra .6V going through my CPU. Even if it doesn't fry it it will mess with the signaling and probably make it incredibly unstable.
But I do like the idea of a water-level LED array. If you come up with a fairly easy way to do it that keeps the circuit isolated from the coolant I'd definatly be interested in the schematic. As it is, I just made a small second window in my case right over my "res" (my 3/8" ID vertical fill tube) and put a blue LED behind that to make a little water-level-window.
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