So, now that I have installed the metal bowl and its gasket from the chinese snow blower carb, and changed the Oring around the tap itself, I filled some gas in the tank. I the vacuum test with the adequate hose and it opens and closes properly. If this cardboard underneath remains dry over night, I am done and satisfied. The leak, by the way, came from the O-ring araound the tap. So the other seals plus the plastic bowl were actually OK. Untill I provoked them, that is.