This O-ring seals the rear axle housing where the axle shaft passes through the brake system assembly. It prevents lubricant leakage from the differential housing into the brake chamber and keeps contaminants out of the axle bearings. A degraded O-ring causes brake contamination and oil loss, reducing braking effectiveness and requiring axle disassembly for replacement. The manufacturer produces this seal to match the original groove geometry in the housing, ensuring proper compression without over-squeezing. On installation, clean the groove thoroughly and lubricate the O-ring with clean axle oil before seating.