Tossing around the idea of building a tractor again. I drew up this feedback linkage for controlling a 3 point setup with a position controlled lever using a standard valve. Nothing to scale, just a diagram to get the idea across. Red circles are hard pivot points, yellow is a "walking beam" type lever, and blue are the pushrods. front of tractor to the left. As lever is pulled back it pushes the top of the walking beam back, angling it and pushing in the valve, which pushes out the cylinder, moving the bottom of the walking beam out and releasing the valve to stop cylinder motion. Won't be able to slam it up and down obviously, but with a fairly quick cylinder on the rock rod it should work fine. Rock rod would be connected to lift arms with chain to facilitate floating, or a single acting cylinder could be used. I like the double acting because I could replace the chain with solid bars to give downforce if wanted. Let me know what you guys think, and sorry about the .jpg, it's hard to see, I'll try to upload the .dxf too.