The automatic relief valve feature limits pressure inside the hub to protect the rear seals from rupture. Since the hub is full of grease, the bearing receives constant lubrication. Bearing Buddy allows quick lubricant level check and ends the need to repack bearings. Constructed of steel. Includes bra to keep grease off trailer wheels. No. 1980 fits hubs with L-44610 1.98" outercup. Pair.