It is an expansion tank, not an overflow hose on your boiler. This happens every time engine gets up to operating temperature. Coolant is not compressible, but expands a lot. If you look into expansion tank you can see how level fluctuate between hot and cold.
Exactly, when pressure surpasses the caps rating it will vent excess pressure which doesn’t happen often.
It is an expansion tank, not an overflow hose on your boiler. This happens every time engine gets up to operating temperature. Coolant is not compressible, but expands a lot. If you look into expansion tank you can see how level fluctuate between hot and cold.