Abstract: A piston for a control valve of a water treatment apparatus includes a body having a central portion from which at least one flange projects radially outward. Each flange has an outer peripheral surface extending around the central portion with an annular groove therein. The annular groove is undercut thereby having a width that increases from the peripheral surface radially inward into the flange. A separate sealing ring is molded into the groove of each flange and is captivated therein by the undercut of the groove. That captivation better enables each sealing ring to resist forces produced by the water flowing through the control valve without becoming dislodged from the flange.