51 Ashton has it exactly right. My 51 half ton started to have gear oil leak into one rear drum. It occured to me that since I was losing gear oil, I should check the level in the differential to be sure I had not lost a significant amount. I removed the fill plug, right after a drive, and heard air escape. That led me to the axle vent, which was compeletly plugged by corrosion and paint. I soaked it in gun solvent, replaced and never had a problem since. I cleaned up the drum but never replaced the seal and haven't had a leak and that was four or five years ago.
