A rigid piston assembly for use in a flush valve, the flush valve having a valve body with a fluid inlet, a fluid outlet, and an outer cover, the rigid piston assembly comprising: a rigid piston having a central opening and a by-pass orifice, wherein the rigid piston is adapted to be mounted within the valve body and is adapted to move axially in the direction of the flow of water from the fluid inlet to the fluid outlet; a relief valve positioned through the central opening of the rigid piston, the relief valve comprising an auxiliary valve and a shaft extending from the auxiliary valve; a top closure configured to connect to the valve body at an end opposite the fluid outlet; a piston seal adapted to fixedly engage to the rigid piston and sealingly engage to the valve body near an entrance of the fluid outlet, the piston seal having a by-pass orifice; and a by-pass device having a body with a first end, a second end, and a passageway positioned through the body, wherein the by-pass device is adapted to fit inside the by-pass orifices of the rigid piston and the piston seal, thereby establishing fluid communication between the fluid inlet and an upper chamber formed between the rigid piston and the top closure, a retaining ring adapted to: a) fixedly engage to the piston seal, wherein the retaining ring comprises a by-pass orifice and the by-pass device is adapted to fit inside the by-pass orifice of the retaining ring; or b) fixedly engage to the by-pass device below the piston seal, characterized in that the piston seal comprises a central sealing ring that extends from the piston seal and is adapted to be placed into an outlet bore of the valve body to sealingly engage the valve body to an entrance of the fluid outlet, and the central sealing ring extends along the central opening of the piston seal and extends down from a sealing surface of the piston seal, thereby allowing the central sealing ring to be placed into the outlet bore of the valve body. a rigid piston having a central opening and a by-pass orifice, wherein the rigid piston is adapted to be mounted within the valve body and is adapted to move axially in the direction of the flow of water from the fluid inlet to the fluid outlet; a relief valve positioned through the central opening of the rigid piston, the relief valve comprising an auxiliary valve and a shaft extending from the auxiliary valve; a top closure configured to connect to the valve body at an end opposite the fluid outlet; a piston seal adapted to fixedly engage to the rigid piston and sealingly engage to the valve body near an entrance of the fluid outlet, the piston seal having a by-pass orifice; and a by-pass device having a body with a first end, a second end, and a passageway positioned through the body, wherein the by-pass device is adapted to fit inside the by-pass orifices of the rigid piston and the piston seal, thereby establishing fluid communication between the fluid inlet and an upper chamber formed between the rigid piston and the top closure, a retaining ring adapted to: a) fixedly engage to the piston seal, wherein the retaining ring comprises a by-pass orifice and the by-pass device is adapted to fit inside the by-pass orifice of the retaining ring; or b) fixedly engage to the by-pass device below the piston seal, a) fixedly engage to the piston seal, wherein the retaining ring comprises a by-pass orifice and the by-pass device is adapted to fit inside the by-pass orifice of the retaining ring; or b) fixedly engage to the by-pass device below the piston seal, characterized in that the piston seal comprises a central sealing ring that extends from the piston seal and is adapted to be placed into an outlet bore of the valve body to sealingly engage the valve body to an entrance of the fluid outlet, and the central sealing ring extends along the central opening of the piston seal and extends down from a sealing surface of the piston seal, thereby allowing the central sealing ring to be placed into the outlet bore of the valve body.