This small brass alloy vent performs a major role in your axle assembly's well being. It vents outward pressure inside your axle developed by turning components allowing oil seals to perform correctly. It also seals itself when submerged to prevent water and silt ingress into your axle. It does this by an internal ball and seat. To check its condition unthread the breather from the axle and shake. You should be able to hear the ball rattle inside, indicating it is moving freely in its seat. Always check, or replace when replacing a pinion, or hub seal to avoid repeat seal failure.
FITS: Series I - III 1948 - 1980 all models except 101