JD closed center systems are very complicated. My advice to you is acquire a JD 4430 tech manual & study it diligently.
I couldn't agree with you more, I have been gathering gauges and o-ring fittings, building a hyd test kit, I had a JD 4430 service manual but it was a b/w copy, gray pressure line pics are not very helpful. that manual is now a door stop.
just bought a new serv manual from deere. fixed several issues, figured out how to isolate a few things, building a pressure test chart, and when I get the numbers I intend to come pester you and the forum again, because I value your experience, and there is only one way to get that experience and that is hands on.