Low flow from top wash arm - Maytag MDB6000AWB

Not enough water flow from pump to rotate the top arm. Dishes do not get clean and soap is not all used. I've removed the filter screen & arms to clean them but they were not dirty. The pump works but not well enough. What parts are most likely to fail in the pump assembly? I found the diagram on Sears website. I see there are several o-rings and a check ball (among other things) are any of these parts known to be problems? Which ones? I would like to order up the equivalent of a "rebuild kit" and fix the pump in one shot. Thanks