Thanks for the help everyone. I was able to get them together correctly and now both valves (tractor has 2) have detent in all 4 positions: up, center, down, and float. And no more leaks! I hooked it up to my haybine to test them, and they work as they should: when raising, the valve lever locks in raise position until the haybine cylinder is all the way up, then the valve trips back to center. Same with the down position.
Part of the reason I'm posting these details is because I looked through the archived posts on the subject, and I could not find a detailed description of how the detent part of the valve goes together. So for anyone in the future who is wondering how to do this, here's my best explanation:
Clamp spool vertically in vise, with flat end where lever connects in jaws. Slide parts 16 and 17 onto top of spool. Install new o-ring, 14, onto part 10. Insert new piston with o-ring, 8, into part 10. Slide parts 11, 12, and 13 onto part 10, then while holding spring compressed with your fingers, screw part 10 assembly onto top end of spool in vise. Tighten part 10 with a small punch inserted into one of the holes. Dab some grease on detent balls, 6, and insert them into 3 holes around part 10. Slide part 7 into back end of part 15, then slide this assembly over part 10, which is now connected to the spool. Make sure the detent balls stay in position as part 7 slides over them. Now insert ball, 5, and ball seat, 4, into top end on part 10. Then insert spring, 3, and screw in plug, 2, while pushing down on spring to compress it. As a starting point before adjustment, note position of plug, 2, before disassembly and tighten to the same position. Screw cap, 1, onto part 15. Replace o-ring, part 9, at front of valve block. Then insert spool assembly, parts 1-18, into valve block and tighten part 15.
Hope this helps someone in the future who had the same question I did. Once again, thanks everyone.