Wow, that brings back memories.
I had a '76 YZ125X which had the same forks as the 250 and 400.
Never could get them to work right!
You are correct, there are no springs. Each fork leg has two chambers. If I remember correctly, the top one on the center is for higher pressure. The other one is lower pressure. I really don't remember what the pressure should be. Maybe try 10 - 15 psi in the lower pressure and 30 - 40 psi in the high pressure? Sorry, don't know the metric conversion.
It is hard to get the left and right side pressure to match equally.
For oil, I also can't remember, but I would drain all the oil, compress the fork and start with 125mm from the top with the forks compressed. Just a guess - that usually works on non-air forks of that era!
Good luck. My '76 125X was a good bike for the day, except for the forks. The '76 air fork is interesting to look at, but it is not so good on the track.