Have you checked to be sure the drain hose is not partially blocked?  If it is clear and there are no obstructions in the drain valve outlet or where the machine connects to the sewer the drain valve itself is not working.

WK78 

I agree with Steve's comments, it was very important to use the proper drain hoses on KA DWs, especially on the KD-20s and earlier.

One of the BIGGEST service problems that KA had with their DWs was poor draining caused by undersized drain systems and overly restrictive air-gaps. KA brought a lot of these problems upon themselves because they were the only DW maker that did not supply a drain line with the purchase of a new DW, most installers tried to just use the drain line from the old DW being removed and in the case of GE DW for example they used a drain line that was too small for a KA DW. KA DWs did not come with a drain line included till the KD 24 models came out.

KA was loosing a ton of money on warranty calls by the early 1980s because of drainage problems and claims for damage to porcelain door liners and tanks. They also had plenty of problems with the wall ovens and cook-tops they were selling after KA bought the Chambers line. It is one of the main reasons that KA sold out to WP, the company needed much better cost controls and management for the consumer side of the business.