A diaphragm type flush valve for use with toilet devices such as urinals and water closets that includes a valve member, a diaphragm, a guide member extends downwardly from the diaphragm, and a disk positioned on top of the diaphragm. The disk is threadably attached to the guide member and functions to secure the diaphragm, the disk, and the guide member into the valve member. The disk includes a rigid body made of a composite material and a seal member made of an adhesive and a thin layer of rubber, with the seal member having a portion on an upper surface of the disk for sealing contact with a relief valve. The disk also includes a plurality of spaced protrusions and inwardly directed on its upper surface in order to add accuracy and stability to the diaphragm assembly.