4020 3 point trouble

Are you saying the hitch is still raising normally in LD position and is lowering too slowly? Are other hydraulic functions working normally? Usually the rockshaft loses ability to lift in L position when linkage is worn or out of adjustment. Losing function in D is unusual. If leak at load control shaft and raising/lowering problems started at same time, you could have a broken load control shaft. There is a bushing that has an oring in it that helps the control shaft seal to last longer. The other things that help is to replace the shaft if has any wear and also replace the bushings and front pin in drawbar support. Then be sure to shim the drawbar support so it has no side to side play. You also could have problems in the control valve or internal linkage. I would repair the leak at Load control shaft to be sure shaft is ok. Also look in bore of transmission case when shaft is removed to be sure the fork that straddles shaft is not broken.

In the LD position, it raises normally, but when you lower it to a set position, it does not maintain that position, but slowly bleeds off.
When running say a bushog, I lower it to where I'm cutting say 8 inches above the ground, and in just a few minutes, I'll be digging dirt. With nothing hooked up, and hitch in the fully raised position, it will stay there. But as soon as I shut the tractor off, the hitch starts bleeding down.
All other hydraulics are fine. The hitch in L, or D, has never worked since we bought the tractor 30 plus years ago. Raise the hitch up fully, move selector to either one of those settings, and the hitch falls as if there was no hydraulic oil left in the lift cylinders.
The load shaft leak, didn't start till 3 months after I noticed the bleed down problem. So I'd say the 2 are unrelated. It has been 15 years since I replaced the load shaft seals and bushings, so I had already planned on replacing the shaft as well. I hadn't thought about drawbar support bushings and pin, so I'll be sure to do that also.
Thanks for your input, jd110.

If the hitch is leaking down with no weight on it with engine off. there has to be leakage in one of about four places. The piston seal, the thermal relief valve, pressure valve or return valve. pressure and return valves may be worn, damaged or out of adjustment. The piston seal is easy to replace. Lower hitch all the way and then remove oil line going to piston cover(1st diagram. then remove the four bolts holding the cover on. The piston is the exposed and can be pulled out. You might lose about a quart of oil when the piston(second diagram#16) comes out. The thermal valve is located under a small plug in valve housing cover. (third diagram plug #1 and valve #6) Remove the plug and use a small magnet to remove any shims, spring and valve. The valve has a tapered seat that is usually totally worn out. Unfortunately the seat in the valve cover is usually worn also. The way I usually fix these is to get a new valve and drop it in the bore in the cover. Then use a narrow punch and a hammer to tap the valve sharply into the housing several times. This will usually reclaim the valve seat so that it will seal. I also replace the spring and add an extra shim or two. After everything is reassembled, raise the rockshaft and let the engine run several minutes. feel the cover where the thermal valve is located. See if you detect any heat build up there. If the valve is leaking, it will heat up and if it is sealing, it should stay at air temperature. If it is still leaking, you may need to replace the cover due to the valve seat being beyond repair.