Yearly(?) maintenance would be to change fluid and filter, Owner's Manual suggested 2yrs/24K miles.

You might even after changing pan fluid, disconnect the upper oil line and run an adapter to hose to drain pan and let the trans pump out the oil from the torque convertor. See bubbles, stop, refill pan with 4 qts and do again. Keep doing this until oil is clean from hose. Reattach cooling line, add fluid, start up and run through the gears, and check level again. If you get a pint or more above full mark, don't sweat it.

The reason I said from the top tank fitting is because GM fills the cooler from the bottom. You case may be different.

Adjusting anything would be non-existent, I believe. Trans may have a shift kit in it. Light throttle is smooth shift, half throttle is a crisper shift, and WOT shift are sharp for OE set-up.

 

I would not put 24000 miles on my car in 20 years so I don't feel bad that I haven't changed anything. I will however drain the fluid and change the filter.
I guess just sitting will be as bad as a ton of driving.
So what you are telling me is that I can't adjust the shifting without taking the trans apart. I am a newb when I comes to older auto transmissions.

 

If it has an adjustable modulator, you can turn the screw inside the vacuum nipple counterclockwise to make the shifts softer and earlier.
You can also remove the modulator, and install a small spring on the modulator valve to further help.

 

Does the 350 have the same thing?
Do you have continue to adjust until you find the right place?
Would you ask for a modulator spring?

Thanks.

 

It's really tempting to jump in here,,,,, nope, won't do it.

Oh hell, yes I will! From the day my car was new, its TH400 hit second gear like a hand grenade going off. Bam! Built that way from the factory. And rubber was deposited on pavement and tire smoke was a hazard to anybody behind me. (that's second gear, not first!) Waddya want a smooth shift for?!

 

Does the 350 have the same thing?

SAdjustable modulator
YES...
I have just done mine..after swapping the 2500 stall for a low 1200/ 1700 stall
1st make sure the kickdown is adjusted correct...full throttle and the kickdown should also be pulled full out and a little bit of play...
The adjustable modulator has a small flat head screw inside as described above
Turning in about 2 turns will up the rpm kickdown about 400 to 500 rpm.
I think it works by upping the piont of change in relation to the engine vac, because before it would kick down about 5 or 6" now it kicks down about 12" ( I have a perminent in car manifold vaccuum gauge.)