an improvement is provided for toilet flush valves on tanks which contain insufficient water to flush properly , which leads to waste of water by double flushing . the improvement enables in - field modification of the flush valve to substantially completely empty the tank . a closing delay device which includes a cup , is attached directly to the top of the tank ball for easy installation , and is tall enough so that when the tank ball is pivoted to the fully opened position the cup is pivoted to a side of the pivot axis opposite the tank ball to delay closing of the flush valve . the tank ball is formed of soft rubber and has an upwardly extending tubular extension with an annular groove therein , which closely receives an annular flange at the bottom of the delay cup device .