A1/8 inch air cock.
In my experience collecting stationary and marine engines, one problem I have encountered repeatedly is missing or leaking priming cups. I have taken the old priming cups and lapped them, tightened springs, changed springs, added washers, etc., but they still leak. I have ordered new ones from a supplier, but these leave a lot to be desired.
If anyone else out there is having this problem, go to your local hardware store and buy a 1/8 inch air cock. If it is for a vertical application, get the 90 degree brass elbow to go with it. Screw the cock into a 1/8 inch sleeve which can be chucked into your lathe. You can then shape it into a pretty respectable looking priming cup. I've put two of them mounted on my 8 HP Nadler marine engine.