I've had the same leaking problem for one whole year now... I made a search using "kitchen-aid dishwasher leaking" and it drove me here...
When I saw the loosen screws on pump upper casing, I went in and saw that, not only one screw was missing (I found it under the skirt, close to the lower pump casing...), another one was very loose and the gasket was coming out of the casing.
...
I've put back the gasket in it's groove, having it turned 180degrees from it's original place in order to put the difformed part towards the rear of the machine... And, also, I've put back the screws back, using some Loctite 518 gasket former on the threads in order to "lock" them in the casing.
Thanks again guys, you saved me a lot of money.
Michel