Not quite sure I've repaired anything or needed to.
I say this because after going through all of this I did not find any obvious failing component. With the kit from HF I replaced all the o-rings with the exception of the outer "tank" part of the jack. After replacing both valve rings I had a very obvious leak from one of the valves. This was after replacing the o-ring with one from the kit that was decidedly thinner. I then put both the originals back in place.
A quick test without a load has it working properly but if I had to do it again without an obvious leak I would drain, refill, drain and refill again to clean any debris and then go from there if necessary.