I have an Adorama two reel tank which is similar to the Patterson. After I dump the developer I just fill it up with water, cap it, and shake it like there's no tomorrow for about 5 seconds, then dump the water. That will do a pretty good job of diluting the developer and getting it out of the nooks and crannies so it stops acting on the film. I do this twice more but agitate (pick it up and shake it) for about 10 to 15 seconds. I follow this by a two minute running rinse. Then you're ready for fixer. The trick is to get that first rinse water in the tank quick, agitate hard, then dump it quick. The developer distribution in the tank is uneven after you pour it out, more is clinging at the reel edges and if you don't agitate well this will be less diluted and cause the uneven development anywhere it is more concentrated. When I first encountered the problem I changed my developer agitation routine several time during troubleshooting but it had no real effect. After studying the pattern of uneven development more inadequate rinse was the only cause that made sense.