I had a similar issue with my 3320, from the drawing posted it isn't identical but I'll offer you my advise anyways. After making sure it wasn't a linkage disconnected or sheared pin I went after the rock shaft valve body. I removed it and blew compressed air in the ports on the tractor side. This should raise the hitch(without load on it of course). So from that point I knew the piston that pushes on the lift arms was ok. In my case I had the PB sintered filter break apart and small pieces ended up lodged in rockshaft valve it self. I was able to take the valve apart and clear the debris. Hope this helps you.
Matt
Matt