a flush valve has a valve body having an inlet and an outlet . a piston is disposed in the valve body between the inlet and the outlet . a piston cap is disposed within the valve body and has a wall along which the piston is guided . a groove is disposed in the piston cap wall for equalizing pressure on sides of the piston translating therein . the groove has a shallow , semicircular shape whereby a possibility that fluid flow therethrough is diminished is minimized .