Expansion tank, I guess. I wanted to be clear it's not part of the pressurized system but there's a hose to the bottom of the expansion tank so coolant is meant to be drawn back into the radiator.

I see green stuff below the heater valve which could be coolant residue. I was thinking there's a small leak in the valve slowly seeping coolant when hot then allowing in air as the system cools. I suppose a cooling system pressure check is in order but will it find a breach that only manifests when the coolant's hot?

Sixto
87 300D^2