Well, I reviewed the RPS troubleshooting section in my 2470 Case service manual and your problem is not listed ?? I believe the 2470 and 1370 RPS transmissions are quite similar if not the same so this manual should be of some support.
You said you performed gear pressure checks and also checked all valves accessible from the outside !!! Does this mean you checked oil pressures at each of the C1, C2, C3, C4 & Lub test ports? If so, are the pressures the same thru all RPM from idle to full? There should only be about 5-10 PSI drop at idle, from the spec of 190-200 PSI at PTO RPM.
Also, when you say the "valves checked good", does this mean none seemed stuck? Did you check the return springs size and poundage? I had problems with 2nd & 3rd in my 2470 RPS and the problem was due to weak springs on the clutching pedal spool and 3 of 4 of the pilot spools. They were about 1/4" too short and 5 pounds too weak.
My suggestion would be remove and rebuild the RPS control valve assembly by thoroughly inspecting/cleaning and installing new seals, gaskets, O-rings and suspect bad parts.
If you don't have a way to test the springs I have the proper apparatus and offer to to it for you.