Diff oil out the vent, why?

I have a 742 pig with 3.90 gears built new for me 12 years ago. Installed it back then with gear lube. Just now getting the car to the driving point. It leaks gear lube out the vent cap. It is not over full. If anything, maybe a little low now. The car has maybe 10 test drives on it, and every time I have gear lube coming out the vent cap. Any ideas why?