In a disc-type stop valve, a valve disc located at the lower end of a rotatable valve stem takes the form of a solid cylindrical metal member having an annular groove formed around the periphery of its lower surface in which an annular resiliently deformable plastic seal ring is disposed. The seal ring is held in place by a smaller-diameter circular metal retainer plate which is secured by bolts against the lower surface of the valve disc and against the underside of seal ring. When the valve is closed, a circumferential lower edge of the seal ring bears against a conically-shaped annular metal valve seat surface in the valve body. A large-diameter resilient compressible conventional O-ring is disposed in an annular space defined by the rear side of the retainer plate, the annular inner edge of the seal ring and a chamfered edge along the intersection of the lower surface and the side edge of the valve disc. The O-ring operates, when compressed, to effect a seal in a space between the seal ring and the retainer plate valve disc without requiring maximum compression of the seal ring. Furthermore, when the valve is in operation and partially cracked open, fluid at high pressure is prevented by the O-ring from getting behind the seal ring and forcing it laterally outwardly further increasing the possibility of leakage fluid leakage. A vent hole for pressure relief extends through the valve disc and communicates with the space between the valve disc and the seal ring to insure inner and outer edges of the seal ring are at equal pressures.