I agree, Al.  I don't see how that much heating of the coolant could happen in a second or two.  I personally think it has to do with lack of any air in the system as it does not do that when there is air present.  Could the fact that I used stainless steel braided lines for the coolant system minimize expansion?  In any case, I have flow with that condition for several hundred hours with no apparent ill effect, so it does not appear to be anything detrimental to the operation of the engine.

Ed

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