Tee fitting? Well to my thought the leaking fitting would not be so intermittent but a sticking valve would be, I suspect as the air pump wears that it sends debris into the lines and it can block the valve seat- sometimes. And since the manifold valve body is nearest the pump that would be my candidate for replacement. Hopefully learn more today..I just dont feel that a leaking tee fitting junction would at times hold perfectly well as this thing is now behaving.