If you depend on conductivity between a probe and ground via the coolant, keep in mind thatbecause of salts and ions (there's metal in that coolant), and high humidity, you'll have a substantial leakage path up the rubber insulator, down the plastic walls, to and through the coolant, and to the block.

I'd bet that the coolant won't carry the current you need for the relay coil, but you could amplify it using an FET (very high input impedance).

How about doing something with the cap? If you screw up, a new cap is cheaper than the expansion jug.

How about some type of optical sensor, much as the Delco (and other) batteries had the "magic eye?"

How about a float (inside the jug) with a magnet that activates a reed switch or hall sensor outside the tank? No holes......

--
Don Foster (near Cape Cod, MA)