Just to make sure I’m not missing something. Our 1968 4020 seems to lose prime of the hydraulic system after being idle for a while. I don’t see any major leaks and no oil leaks. A couple of years ago it started to happen that after starting you had to wait 10-15 seconds before anything could be done with the loader. Now it can take a full minute before any hydraulic system works.
Does this mean the bomb is on its last legs or is there something else it could be missing?
Here’s a service bulletin for adding a check valve to eliminate hydraulic oil drain causing this problem you mentioned.
Why not check the pressure potential of the pump using a hydraulic gauge and see if the relief is properly adjusted and releasing at the set pressure?
Check all the fittings and the suction tube, you may have a suction leak.
Have you looked in the tank to see if there are small bubbles.