Cup seal for a master brake cylinder

In a master brake cylinder comprising a housing defining a bore therein, a piston reciprocally mounted in said bore for movement along a longitudinal axis thereof and an elastomeric cup seal disposed in said bore to be reciprocated along with said piston and having disc shaped base portion integrally connected at its periphery to a flexible annular flange portion thereof disposed in sealing relationship within said bore, the improvement wherein said annular flange portion comprises means formed thereon for inducing rotation of said cup seal relative to said housing upon reciprocal movement of said piston, said means comprising a plurality of circumferentially disposed grooves formed on a periphery of said flange portion with each groove extending generally in the direction of said longitudinal axis from an open end thereof disposed at the junction of said base portion with said flange portion to a termination at a closed end thereof disposed between the axial ends of said cup seal.