JD, those valves should unscrew from the transmission case. You may have to find a 6-point socket that engages completely with the hex on the valve.
I had this problem with my 318 when I got it a few years ago. As I recall, unscrewing the valves was no problem, and I just soaked them in Dexron and worked the plungers until they freed up. Stuffed some paper towels in the holes to keep any dirt from getting inside.
I had the hydraulic system open on that tractor for about a week, as I had leaks elsewhere to fix.
It had type F in it, which I drained, and was also able to work the two cylinders on the tractor to empty them. After I reassembled things (with a new filter), I refilled the system with lo-vis Hyguard, bled the system, and it's been fine ever since. The small amount of type F left in the system colored the Hyguard a bit, but no need to drain and flush again.