I experimented with this on my KDS-18 and and had very poor results. Next, I made 2 attempts at jamming a rubber stopper in the upper rack water orifice tube against the back wall. It blew it out immediately on both occasions. Following that, I tried jamming the stopper backwards (It took every ounce of strength I had). That lasted about 4-6 seconds on all 3 attempts before shooting it out again. Threw in the towel.
If one has the space, I recommend purchasing a one arm wonder KD-17 or older unit and removing the upper rack.
If one has the space, I recommend purchasing a one arm wonder KD-17 or older unit and removing the upper rack.