When the gearbox is flipped upside down the air valve must be plugged. I realized that the little steel tube holding the air cap was not very difficult to rip of or knock into the housing. Not good. So I solved it by threading the hole with a standard M12 tap, made a new tube with an oversized M12 thread in one end and a smooth 12 mm dia in the other, with a cut for a large screwdriver. I filled the thread with black silicone and worked the tube into the hole. It will NOT get away from there.
To make services easier I mounted a plumbers valve. It's the high qually brand Ballofix and I happen to know that they survives decades of oil abuse. Now it's just a dust cap left and I can change oil with racing style... :-)