A flush valve includes a body having an inlet, an outlet and a main seat assembly between the inlet and outlet. A piston moves within the body and with the body defines a pressure chamber which is used to normally maintain the piston closed upon the main seat assembly. There is a refill orifice in the piston connecting the pressure chamber with the inlet. A relief valve is carried by the piston for venting the pressure chamber and there is a fluid driven apparatus for opening the relief valve, which apparatus is attached to the body. A hydraulic bypass in the body is connected to the fluid driven apparatus. The main seat assembly has a plurality of peripherally disposed fluid passages connecting the hydraulic bypass and the inlet to provide filtered fluid to the fluid driven apparatus. A first seal on the main seat assembly has a flat portion and an arcuate portion constrained within a channel. Lockout lugs formed in the seal prevent installation of an incorrectly sized piston.