This was a very interesting discovery in my restoration of the rear axle. I just naturally thought that the vent would have a large hole from one end to the other. But nooooooo! There is actually an orifice halfway down the length that was totally plugged with grease and dirt. Using brake cleaner and a small wire I was able to dislodge the debris.
After enlarging the photo, I noticed that the hole still had some blockage so I used a 1/16" drill bit to remove the rest of the blockage. Was this blockage the cause of the pitting of the gear teeth and roller bearings? Perhaps. Did the heat and/or condensation which had no escape path cause leaking seals? Possibly.
This was one of those easy to overlook details that make the restoration process a challenge.
