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Push the seat back forward and you'll see the stops for the back. Rubber bumpers on the back and metal stops on the seat bottom. The bottom stops are best for the adjustment, just screw em up a few turns.
You also might check to see if the rubber on the stop isn't shot. I replaced all of mine from Ecklers.com. They were about 7 bucks each if I remember correctly. Large phillips screwdriver takes them out. My seat were sagging also and once I replaced them, my seats sat up more than an inch...DDBS