A high pressure metering pump employs a flexible diaphragm extending radially from a piston as the moving pump element. The diaphragm is secured by an annular anvil at its periphery to form a high pressure seal, and the center of the diaphragm is driven reciprocally by a mechanically driven piston. A support assembly for the diaphragm includes a moveable metal ring concentric about the piston and an elastic, resilient ring concentric about the piston and interposed between the metal ring and the diaphragm. The elastic resilient ring is formed of a material such as neoprene, Viton, or the like to mediate and distribute the force between the diaphragm and the metal ring. This assembly allows the diaphragm membrane to support the high pressures that are prevalent in HPLC.