Bingo! I never use the cork gaskets. It's too easy to tighten them too much and they will leak. Been there and done that. Use old one or go with a Felpro gasket.

The fluid exchange sounds good, but I don't have the proper tools to do it. I decided to drop the Dex VI, every 20,000 miles since the 6T40e filter is not removable. Most are fully synthetic, but I saw nothing on a bottle of STP Dex VI claiming that its full-synthetic at all, so it's Valvoline or GM Dex VI which are fully synthetic Dexron VI fluids.

 

as long as you change it sometime. I do like the oem fluids.


I remove the cooler lines....and let the tc pump out the old fluid. I have made a trans filler from a weed sprayer (virgin one) so I pump it in as the tc pumps it out. you could shut the engine off every 2 liters and refill the trans....I'm not sure where the lube circuit is feed....before or after the cooler....if its after the cooler... lube circuit wouldn't get fed until your done.
I have done this on my ford and dodge using the refill method and both trans are running perfect.

I overfill one liter...run and drain 2 liters approx. 10seconds run time...shut down...fill 2liters...repeat until I have exchanged the complete capacity

 

Fluid goes through the cooler, then to the lube circuit.

 

Update on progress:

I pryed on the y-pipe and almost had the pan out. Decided to not try to loosen Y (really need taller front stands),
and go to plan D.



When the pan was hanging I was able to drain maybe 2 quarts. Reinstalled pan w/ whatever gasket was stuck to pan.



Then I removed the trans cooler line placed metal and rubber tubes into a bucket started engine fluid went everywhere then I quickly realigned bucket turned off engine when the fluid started sputtering, let drip then restarted a few more times (4) letting it run maybe 10 seconds each time I don’t think it ran dry at least I hope I didn’t.

I didn't add flud while it was being pumped out.



Probably drained out total 6 quarts of nasty black fluid however there was a little redness, added a tad less than 5 quarts.

Plan to let the fluid go down into pan before restarting engine.



I think this trans holds 16 quarts and I was only able to drain 6 even when pumping it out.



The rear dif. Fought me for a couple days the lowest cover bolt was totally rusted to cover and the outside head of bolt basically crumbled I grinded a grove for a flathead screw driver but ended up using the grove to beaton w/ a hammer and screw driver.



I need a rest and research to attempt the front diff.



the transfer case was easy compared to trans.

Thanks Guy’s for advice TB