A valve comprising a valve body having a first end, a second end, and a through opening forming a fluid flow passage and a wall surrounding a portion of the fluid flow passage, the fluid flow passage defining an axis that is co-axial with the wall. A valve element is rotatably mounted in the valve body, the valve element having an annular, radiused sealing surface and being rotatable around a second axis perpendicular to the first axis passing through the fluid flow passage. A valve seat assembly is disposed in the valve body for sealing engagement with the sealing surface of the valve element, the valve seat assembly being axially displaced from the second axis toward the first end of the valve body, the valve seat assembly comprising a seal ring of a resilient, plastic material and having an annular radially inwardly facing first surface for sealing engagement with the sealing surface on the valve element and an annular, radially outwardly facing second surface, an annular radially outwardly facing groove being formed in the second surface. An elastomeric backing ring is disposed in the groove, the backing ring being circular in cross-section and having a center-line defined by a first plane passing through the center of the backing ring perpendicular to the first axis, the engagement between the sealing surface on the valve element and the first surface on the seal ring being substantially annular, line contact when the valve element is in the closed position and in the absence of any fluid pressure acting on the valve element, the line contact engagement lying in a second plane axially displaced from the first plane in a direction away from the second axis toward the first end of the valve body. There are also provided retaining means for retaining the valve seat assembly in the valve body.