I have a Ford NAA Jubilee that has the same problem. As soon as you shut off the engine the lift slowly goes down. In the winter it will take a long time to warm up before it raises.
Whenever I look at that top lid and think how heavy and a pain in the butt it is to take off, I just say I can live with it for now.
I had the tractor for 30 years and its time to move it on down the road. Looking at a Massey 1540 with FEL. Boy, will I feel like a pig in S*** when I get that.