Info update
Had some time this afternoon to run a few tests. Here's what I found:
When I switched the boom and bucket hydraulic lines, the boom will lift and hold the tractor up. So the issue seems to be in the SCV boom down valve. I also noticed that while they were switched, when I raised the boom, and then curled the bucket, it (the empty bucket) would start to dump. That wouold be the valve that is normally for the boom.
When I switched the lines back, I did the test again. And at first, it also held the trator up, and I was thinking what the heck is going on, it's working. Then I put the bucket and boom to both extremes, and held for several seconds each end of travel, to feel sure that any trapped air would bleed out. After I did that, I lifted the front axle up again, and it dropped right back down in just 2 or 3 seconds. Now I am even more perplexed...
But I noticed that as the tractor went back down, that the tractor was also moving (none of the previous tests had the brake set. So I held my foot on the brake, and lifted the front end again. Now it will stay there. Remove my foot from the brake, and it lowers the tractor as it rolls the tractor backwards.
Should I expect the boom to hold the front end up whether the brake is set or not?
I also tested the hydraulic pressure at the port for the boom down line, and it was 2300 psi. Don't know if that is the way to test it, but at least I know that at 2100 rpm, I have 2300 psi when the SCV valve is held down.
I did try to see if the SCV valve was returning to neutral, or overshooting and going slightly past it to raise the boom. It did not appear to, but I will check it more closely tomorrow.
Thanks for all the suggestions.
This is how I plumbed up the fittings today.
PSI at 2100 rpm, with gauge in-line with Fel. I did not do a dead-head test yet.
This is the hydraulic fluid that I have in the tractor. In the print at the bottom, it lists as meeting the specs of John Deere, and is compatible with both synthetic and regular hydraulic.