The expansion tank, pressure cap, was my first thought and so I rigged an overflow lower than the expansion tank so I could monitor. Not a drop.

I hadn't thought of the pressure differential in the heat exchanger. One fact I failed to mention is that it never leaks when the engine is not running. I have filled the expansion tank right to the top, higher than the heat exchanger, and left the raw water inlet open for days and no leak.

I'll have to give that more thought.

Thanks, Eddie