A drainable radial diaphragm valve includes a valve body that defines a valve cavity, and an inlet passage and an outlet passage. The valve body may include a valve seat at an end of the inlet passage, where the inlet passage joins and creates a flow path into the valve cavity, and a port at an end of the outlet passage, where the outlet passage joins and creates a flow path from the valve cavity. A surface of the valve body may slope from the valve seat downward to the port of the outlet passage. A flexible diaphragm including the protruding boss may be mounted with the boss aligned with the valve seat to contact and seal the valve seat, and close the flow path from the inlet passage into the valve cavity when the diaphragm is flexed to axially displace the boss toward the valve seat.