The important thing is that; whatever the cause, the pressure transient is apparently a non-issue.    Certainly lines covered with braided SS aren’t going to expand much, but they still have flexible walls in there that do distort bit with pressure, as will the thin-walled tank on a radiator.  Clearly there is a hydraulic pressure being transmitted through the cooling system.  My guess is that the cause is likely the immediate expansion of the inside walls of the rotor housing (temperature and pressure) while everything else is cool, followed closely thereafter by initial heating of the coolant.  Heat transfer through the rotor housing does not happen instantaneously.  It’s a guess, but hey; other than for mental gymnastics, who cares?

Al


I agree, Al, inquiring minds at least like a plausible hypothesis and I think the discussion has led to that.  Perhaps I'll leave the "cup" of air, just for peace of mind {:>)

Ed