I have followed this thread with some interest and I want to make a few points from observing the pictures....
1. This control valve is of an unusual type (very old design), IMO, it is more of a sandwich type CV rather than a stack valve.....because of there are to many layered segments in the valve...
2.Pump and return ports are at the same end (under) of the entire sandwich...that make me think the valves might be straightly plumbed in a series, eventually we are dealing with a closed center system (which I doubt)??
3. Extending the the two parallel arm-cylinders is depending on gravity force more than force/flow from pump (over running)....with that "heavy" attachment, cylinders should extend synchronized, (if there is nothing mechanical that sticks in linkage or friction in cylinders) as long as the is free drainage of fluid from the rod side of cylinders...
Question 1 to OP: Does the operation of cylinders act the same with heavy attachment hanging in the arm as with out attachment load in the arms???
4. Ability to lift the arms synchronized will depend on load on each cylinder and/or back pressure on the capped side of cylinder. If there is uneven load or back pressure between the cylinders, control valves for each cylinder needs to be feathered differently to allow synchronized motion.
Question 2 to OP: Obviously your tractor new to you. Have it had this problem since you got it??
I agree with some other posters here, system works as it supposed to work, and I do not think your valve is faulty.
I also think you will have to replace that entire control valve, with a modern 4-spool valve (mono block or stack valve doesn't matter, mono block is cheaper and less to worry about)
Edit:
My point with your valve is that it might be missing this "red" power core and the parallel return core , that connects valves parallel, and makes it possible for valves to operate functions simultaneously
