Missing 3rd on 904 trans

Saw mention in the Motors Automatic Transmission Manual that there could be some association with the oil pump. Bad seal causing some leakage.... It does sound the world like the 2 -3 Valve is not actuating but you said the valve body was all clear. Strange.

I could not get upshift 1 -2 nor 2 -3 and it was debris in the shift valve area causing it to stick