On your truck- how is the piston (controlled by the brake pedal) actuated? On the H80XL I service there's an adjustable rod between the brake pedal & the piston you mention. The adjustment is in the length of the rod.
The rod only adjusts when the trans goes from "neutral" to "drive". It doesn't adjust the transition smoothness. What engine rpm is present when using the brake/inching pedal. It's designed to be used at idle- around 750 +/- rpm.
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