Isolation valves would make life easier but there is a gasket in that joint that has failed. Drain the wate down, unbolt the pump, clean the surfaces, put permatex on both surfaces, put in a new gasket and bolt her back up. Now fill with water and bleed off all air; you know the air is gone when you can't hear water splashing and running in the system. I loved the hot water system we had in our house but it loved gas too much. I now use a geo-thermal unit.