If the Lift cylinders retract when the lift spool is in the neutral position here are some suggestions to check for...

- Cylinder seals leak or check valve for lift spool leaks.
- Large leaks between the spool and bore in the main control valve.
- Hydraulic line(s) leak. (if there are no visible leaks then rule this out and look at the first 2 problems)

There regular controls not electronic.

What a easy way to diagnose a problem. Thanks for the help, I'll give it a try.

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.