Hello Bravo - Harris and Queenslander,
I 100% agree with your approach of draining the back end, taking off the filter carrier, etc., and looking for oil entering the back end, if Queenslander thinks this is the way to go, but would urge caution if this means running the gear pump "dry", as gear pumps should be operated with a full supply of oil in them. This is pointed out clearly in the manuals I have.
If this proposed test does not lead anywhere, and I know this is going over old ground, but honestly, despite everything that has gone before, I would suggest the combined steering brake and clutch / scavenge pump is the next thing to eliminate as a suspect. I know you have told us the pump is new, however, the following are the facts as I see them.
1. Queenslander has had a similar fault (oil draining from transmission and raising level in steering brake and clutch / bevel gear housing). In his case the transmission oil was entering the steering brake and clutch circuit through the pump. His pump configuration is the same as yours.
2. The exact symptoms are identified in the Fiat Allis 10-C troubleshooting guide (page 42 item 12 as attached). The suggested causes are:
(a) clogged transmission screen (this has been addressed by your screen cleaning exercise)
(b) defective scavenge pump
3. New parts do not always work correctly. You have to be unlucky to cop this, but unfortunately I speak from experience, having been on the wrong side of the pachinko ball of life in that regard.
4. No disrespect to anyone, but the installation process of new parts has been known to be the source of the problem. Again, it has happened to me.
My reading of the facts may not be applicable to your situation in the end, but eliminating possible suspects will enable you to get one step closer to solving the problem. All the best.
