A means for mounting a valve assembly in a plastic faucet cover plate and for securing said plate to a deck of a sink. The means comprise an annular seat (75) formed inside of a flange (74) in the cover plate (70) in which the valve is disposed, a plastic split annular retaining washer (60) adapted to be seated on said annular seat, and an annular flange (32) having an abutting surface on the bonnet (14) of said valve assembly (11) adapted to abut against the top of the retaining washer and press it against the seat when the valve assembly is inserted into the flange of the cover plate. The bonnet (14) also has improved sealing means comprised of an annular flange (21) having an undercut in its bottom into which is inserted a sealing member such as an O-ring (24). The sealing member is encapsulated on three sides by the material forming the bonnet.