TH400 pan still leaking!

[mechanicalman]

Most important issue on oil leaks is gasket/material quality and surface prep. I would fix the governor cover first as it might be blowing back onto the tail housing. I recommend using a cork governor cover gasket, factory GM is cork. Now the prep-remove the cover and clean it with a wire wheel (safety glasses). Make sure the cover is not distorted, usually if they have been over-torqued the metal under the bolts will protrude towards the trans and the cover will not crush the gasket enough to seal. Straighten the cover, then get a drill attachment that takes a sanding disk, a really small one. Put sanding/grind marks across all the bolt holes on the gasket side to put grip (friction) or "gription" on the gasket to keep it from splitting.

I know, that sounds like a lot that you should not have to go through, but that is how you put that cover on and not worry about leaks. I used to rebuild transmissions in a GM Cadillac dealership, and I had to make them leak-free at whatever effort necessary. You will not find anything like this in the GM service manual, but then again, the manual is for newish, non-damaged parts and the T400 is very old and that cover has likely been tweaked.

Even if I was using a new cover, I'd still prep it. It's a really common leak, and it's a poor design (leak-wise). Aftermarket gaskets for that cover are usually hard, thin cardboard that is not appropriate especially on a worn cover.

I do not recommend using sealer on that.