If the tractor is running, the hitch should automatically re-adjust upward for any settling that may have occurred.
I assume a 4020 JD. There is a cam on the rockshaft and on the cam rides a follower that is connected to the draft sensing linkage. A roller in the rockshaft valve's linkage rides on the follower. If you are set on "D" the draft sensing linkage does not have any effect on the rockshaft valve, only the cam's position can affect the valve.
The valve consists of two balls and seats. One is for raise, the other for lower. The rockshaft valve's roller is pushed back or allowed to move forward due to the cam action as the three point goes up or goes down. Via the linkage that is connected to the roller, the corrisponding ball is pushed off it's seat and allows oil in if it needs to raise, or out if it needs to lower. The cam rotates as it is raised/lowered and when it gets high or low enough, the ball re-seats and flow is stopped. The hitch then maintains that position until it leaks down slightly or until you move the three point lever. The correct ball is then pushed open and the hitch reacts again.
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