Stock rear brake distribution block -- also vent for axle?

That is why there is a loose sheet-metal cap on top of the bolt (at least in my 1965 C-body, 8.75" rear). Confused me until I read, and good I didn't get the bright idea to seal it better. In most cars, the pinion seal probably leaks enough by now to vent air pressure. I wonder if those are worth replacing since it doesn't loose much oil and I think just oil splash (level sits below seal), plus keeps the rear-end from rusting.