Walked out of Pro Rebuilders with a suggested replacement unit. My SI30 is apparently not really worth rebuilding. Too old, too expensive of parts for what the end result is. Cummins gave me a $470 price on a replacement SI36. Pro Rebuilders suggested an SI21 I think it is, which I walked out with for $200.
Bobby of brolite fame has shared some Rockwell manuals with me that he was able to find. 60-65PSI is the regulated pressure for the trans so I've got first steps on that lined up, and about 15 pages of air system manual (it's 30 pages, but it's for a 13 speed. Half of it should apply to my 9) to go through troubleshooting steps on. I'll just need to find means to adapt a pressure gauge to whatever size the small lines are, hopefully there are not a dozen line sizes on the trans but the diagrams indicate there may be.
Bobby's guy says it's likely a top cover issue. That was rebuilt once before on this truck. I'm going to go through service records again and spend some time on it so I can reference this stuff.
Now please, weather, dry out! I have things to do.