embodiment of the end cap of the invention indicated generally at 20 is incorporated in a vehicle wheel bearing that includes a rotatable inner spindle 22 and a surrounding outer hub indicated generally at 24 spindle 22 and hub 24 define an interior bearing space 26 which contains a significant volume of air and lubricant at high speeds internal friction generates heat and outside heat is also conducted into space 26 from the brake rotor not illustrated internal heating causes a pressure rise in space 26 relative to the outside or ambient pressure due both to heating of the air and to gases released from heated lubricant one side of space 26 is sealed by a conventional rubbing seal 28 which does not include any provision for overpressure relief the other side is closed and sealed by end cap 20 which does fig2 shows some structural detail of hub 24 which cooperates with end cap 20 in both its installation and operation a cylindrical outer surface 30 ends in a circular groove 32 outer surface 30 and a coaxial inner surface 34 border a chamfered annular end face 36 a typical hub 24 will already have most of these various surfaces although groove 32 would probably have to be added and inner surface 34 might have to be machined more finely than usual in order to work successfully with end cap 20.