Some of the bolts that hold the tail shaft to the gear box portion of the trans have the potential to leak. They use a special washer that is coated with a black sealer. I used the special washers and mine still leaked. So after my trans was in my car, I backed off each bolt, one at a time, and cleaned and sealed the flange end where it touches the housing. I used ATF safe RTV to seal every bolt. I finger tightened each bolt and waited the recommended time before going back to full torque. I have no leaks from the bolts now.
