Hi all,
So I think I should update my progress as things are much better now, at least regarding the original issue.
Other problems remains, but I will come to that at the end.
Here is what I changed that gives me more satisfying results that works for HC-110 dil 1+66 :
1. No pre-soak
2. Only just enough developer to cover the reel(s). In my case it comes down to 400ml per roll of film with HEWES ss reels in ISE ss tanks.
3. As stated earlier I use now a more vigorous and BRISK agitation pattern. I agitate more, but more importantly I apply more force during agitation, rising the tank slightly and lowering it with just enough velocity to force a downward movement of the chemistry and reels. I use the agitation scheme described in the Darkroom Cookbook as well as by Ansel Adams in The Negative : rotating the tank as I invert it. I also add a swirl movement at the end, before taping the tank to remove air bubbles. I started with kodak recommendation (1st 30sec, then 5sec every 30sec) but that gave me too much contrast and would force me to use tri-x 400 at something like 160-200 iso to compensate for the shorter development time. So I switched to Ilford recommendations (1st min, then 10 sec every minutes), and as long as the first minute agitation remains vigorous (1 complete inversion / second) everything looks OK. I can even slow down the subsequent agitations (4-5 / min) to tame the contrast. Unfortunately, and to answer Chris question, I cannot use semi-stand agitation as it brings back uneven development artifacts. More specifically, overdeveloped, pure white surges along the edges of the image (the edges that are in contact with the reel). This artifact is actually always present but with normal development it's negligible.
4. The USE OF AN ACID STOP BATH. This is the only way I could get rid off the overdeveloped steak that would randomly appear in the middle of some frames as you could see in my last exemple.
This is very unfortunate as I use a lot of staining developers (PMK pyro, Prescysol and Prescysol EF). With PMK, the acid stop bath doesn't appear to be much of a problem and actually seems to lower general stain, although I might be losing some highlight details without noticing it. On the other hand, with Prescysol, Peter Hogan do insist not to use an acid stop bath. But no matter how many water bath I use or even if I agitate the water stop bath like a mad man, I will always end up with some sort of streak mostly visible on evenly dense part of the film . Here is an exemple of a clear blue sky (no clouds) shot @ zone VI developed in Prescysol for 8min with standard agitation (as semi-stand gives me uneven development). I raised the contrast to make the artifact more obvious. It is much less than in the previous exemple but still there nonetheless. If anyone has a clue on how I could get rid of this one without using an acid stop bath...
You can also see the white surge I mentioned earlier on the right edge of the frame. No matter what I do, I can't get rid of this one...
Again, thank you all for your help and comments!
Vania