The disc brakes on the front of all Classic Minis have this large nipple to allow for proper bleeding of the system. The only problem is that after repeated use (typically with a crescent or poor-fitting wrench) the edges become very rounded and thus they become very hard to use as intended. This same bleed screw was used in many applications including the front disc brakes on Sprites and Midgets, on the early slave cylinder, and on the Morris Minor, as well.

See 513118 for small (1/4" diameter) version. See SMP100030 for rubber dust cap sized to fit the large bleed screw.