I replaced all the o-rings in my rear HWH units (Tag and Drive axle 6-packs).
Oddly, HWH only sells two of the tree o-rings needed to reseal these.
I found an article on rebuilding these valves on the old Foretravel site, and used the part numbers he provided. I ordered from Grainger.
I have plenty left over. If you want I can send you some - or if it's easier/faster, order on-line from Grainger (you have to get 25 or 50 minimum, depending on size, if you order from them).