As I read, you rebuilt the transmission. Am I correct? When you rebuilt it was there large amount of grey material in it. If so, The transmission control valve will have deposits in it and has lock open the high pressure dump spool. This dumps all the pressure to sump. Clean the control valve and all the spools. This requires you to disassemble the transmission control valve completely. I will assume this is your problem as it has happened to me. Overseas and USA models my differ, but this is my experience with a 8 series and 7. Good luck, hope this helps.
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