Diaphragm valves include plastic diaphragm and reduced or no elastomer backing cushion, where peripheral or all sealing forces may be applied directly to the diaphragm of the valve assembly. The valve assemblies may include a pressure ring to apply sealing force to a surface of the diaphragm with a reduced size or no backing cushion allowing a two-fold or more increase in pressure capacity of the valve assembly. An elastomer or metallic spring placed above a center diaphragm boss may provide center-to-edge compliance for passage seal.