I ran into a similar issue with my 727 recently. The seal rings on the input shaft were blown out and the transmission acted as if it was bound up. Car didn’t want to go into reverse and with throttle application it would try to move by would just creep. Did the same in forward gears but not as bad. I bought the trans complete from a very reputable torqueflite specialist and when it was put together it had a lock up input shaft in it. The input shaft diameter was smaller in the seal area then the stator and kept blowing the seal out. Both pieces were replaced by a different shop after he found the issue and transmission is Golden now. I too noticed a small amount of fluid in the converter and was told the seal leakage from being blown was directing the converter charge elsewhere in the trans. Guy told me that the blown seals was causing the bands to be constantly applied essentially binding up the transmission.