A seal mechanism (22) for a rotary control valve having a housing with a plurality of ports and a generally cylindrical rotor having an external surface and a plurality of ports rotatably located within the housing. The seal mechanism is located between the external surface of the rotor and the housing and includes a generally cylindrical collar (142) which extends substantially around the circumference of the rotor. The collar includes one or more ports adapted to respectively align with the ports in the rotor and in the housing and a groove extending around each collar port. A seal member (138) having an aperture, an outwardly extending first flange (184) and an inwardly extending second flange (192) is located within the groove of the collar. The first flange has a first sealing face adapted to sealingly engage the housing and the second flange includes a second sealing face adapted to sealingly engage the external surface of the rotor. The seal member is adapted to provide a sealed fluid passage through the aperture of the seal member between a port of the rotor and a port of the housing. A retainer member is associated with the seal member and includes an outer rim having an outer peripheral edge and an inner rim having an outer peripheral edge. The first flange of the seal member extends around the outer edge of the outer rim and the second flange of the seal member extends around the outer edge of the inner rim.