If I understand correctly, I think I can address your problem. I have a TO30 I use with a rotary mower. I have used two different means of maintaining mower height.
1. As described by marlow, if the everything is rigid put the mower on a level surface. (I usually disconnect PTO shaft so I can run hydraulics without spinning blade.) Lift mower slightly, move quadrant lever to stop it from rising. It will slowly settle. Now turn top link to make it longer and apply pressure to large spring. If your draft control mechanism is working, as the spring comes under pressure the lift arms will start to creep upward. Once you find the right place they will hold constant height indefinitely. This works like a charm on level ground. The only problem I ever have is a sudden change in grade that changes the compression on the top link and thus causes the mower to lift or fall quickly. Perhaps a wiser soul can resolve that issue. You can read about this in the archives written better than this.
2. Also in the archives are (vague) descriptions of various position control mechanisms that people have made and added to their Fergies or 9Ns. I have tried two iterations. My current one is made of wood and CPVC. This works great for mowing, too, and has the added benefit of also working for using a carryall, etc. It is essentially a feedback mechanism that pulls the quadrant lever up as the lift arms fall or pushes it down as they rise so that it self regulates.
Both items above require hydraulics that work as they should on a TO20/30. If the hydraulics are inherently broken, that's a different issue.
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