The manual shows a Rockshaft Rate-of-Drop-Lever, located on top of the rockshaft housing in front of the operators seat. Turn the lever counterclockwise to increase the rate of drop, or clockwise to decrease the rate of drop. The illustration in the manual shows the lever positioned parallel with the centerline of the tractor, which I assume is 50% rate of drop. Maybe if the lever is turned clockwise as far as it will go, it prevents the rockshaft from dropping.