The water cooling system seems to have survived its little stuck solenoid incident, but as my running of it up to now only was a test run before wintering, I'll still go ahead and take it offline, so I can fit a proper expansion vessel to it.
I can't say or write "vessel" without thinking of Chekov in Star Trek The Whale Movie (i.e. "Star Trek IV: The Voyage Home"). Permanent damage, that.