On the right track!
There are 2 reasons why the machine would lose water.
1. There is something caught in the drain valve like a toothpick, a straw or some other object that got past the drain impeller.
2. There is a rubber flapper that closes to seal the valve when the machine is running. Those can deteriorate and break off in little pieces or it has simply fallen off the flapper support.
I would suggest that you remove the drain valve from the machine and do a visual inspection of it. That should tell you how to proceed. Clean the valve of the debris or replace the valve.
Those valves are still available from Hobart but just know that any valve used on the 18 thru 20 series can be used to replace your faulty valve. In a pinch, the straight thru style used on the 17 series, can be made to work in your machine too.
Come to think of it, you can even use the older 90 degree valve that was used at one time on the 15 series. I actually used that style on my KDS-18 at one point without a problem.
If you need a valve, I have a few used ones and I am sure others will have them too.
Signing off from Bombay,
SteveT