Abstract: A rotary control valve has a flexible cup-shaped diaphragm ring seat positioned within a valve chamber that has an annular seating lip portion on inner opposite side portions thereof. An annular outer portion of the diaphragm ring is retained against an annular wall of the chamber by an outer portion of a retaining ring member that is longitudinally adjustable along the inner wall of the chamber. The retaining ring has an inner rigid portion that provides a hard seat surface against which one of the annular lips is compressed into sealing, fluid tight engagement as a spherical plug of the rotary valve is simultaneously brought into sealing fluid tight engagement with the other lip. The fluid acting on an inlet side of the valve is applied by way of openings in an annular intermediate portion of the retaining member to the diaphragm ring in a direction that allows the plug and diaphragm ring to make good fluid tight engagement with one another.