Beat me to it, Don; I was going to suggest that. Perhaps you could take that suggestion one step further: drill that hole a tiny bit larger, then insert a split pin through it. Use one long enough so that you can bend the ends slightly to keep it from flying out yet leaving it free to move up and down a bit. That's essentially how the breathers were done on Herald and Spitfire, etc., differentials. Some movement of the pin helps (but doesn't always completely) prevent dirt and greasy grime buildup from plugging the hole and causing enough internal pressure to start forcing gear oil out where it shouldn't come out. (Much of the time, a plugged breather does cause what appear to be "leaks"; just freeing up the split pin by cleaning off the grime often takes care of 99% of the "leak"!)