My buddy Rob recently acquired a 2002 F-250 with a 7.3L power stroke and a 4R100 transmission to tow his enclosed race car trailer. He complained the transmission shifts were very soft and at low speed/high load the transmission seemed to hunt between 3rd and 4th. We removed the accumulator valve body, disassembled it, cleaned it, resized all the bores with the Sonnax bore sizing tool and installed Sonnax shift valves, accumulator springs and a .427" pressure modulator valve. The new pressure modulator valve was about 50% larger than the original and the OEM pressure modulator valve that was removed was heavily worn and starting to discolor.

We reassembled the valve body, filled with 8 qts of fresh fluid and confirmed the shifts were firm and held at all speeds.. ie no more hunting for gear.

Once the transmission issues were resolved, we hooked up an SCT x3 tuner and installed the Generic "street" tune for the 7.3L diesel (rumored to give 60-80HP gain). The truck felt like it lost 500 lbs, the engine was more responsive and it would easily light up the tires from a stop.