TO 20 hydraulics

The cylinder could be leaking or the supply tube to the lift cover could be leaking causing the lift not to work. You can also observe the linkage motion when you move the lift lever. Since you say that the control fork was attached to the control valve, you"ve elimnated that as a posible source of the problem. The best way to check the valve for operation is with the system engaged and the engine running. Move the lift lever upward and observe the motion of the control fork to move the valve forward. Move to the full down position to observe the fork move to the rear. Then move to the neutral position and observe that the control fork moves to a position between full up and full down. Also check for a stuck relief valve. there sould be no noise o turbulence above the relief valve. If there is then ther may be some dirt holding the relief valve open.
That"s all I can suggest.

yes it is called the safety valve assy,,, you should be able to move it in and out from the inspection plate, I believe to remove it and check further you may have to remove the pump, I dont believe you can get a wrench in there to loosen it,, good luck,