Does it have electronic mini-levers or regular hand lever? I use a block off system to determine if I have a valve malfunction or cylinder packing. As old as this truck is, I would look closely at cylinder packing being the issue. Start by blocking off the free lift cylinder. I use a ball type valve to install in line, raise cylinder and close ball valve. If it drifts downward, you have found it. If not move the valve to the lift cylinders, if you do not have downward drift, the control valve is leaking by. If you get drift, one or both lift cylinders is leaking internally.