For your latest symptoms.
From the ATSG. These are your possible problems.
The blue requires pulling the pan.
Yellow requires pulling the pan and valve body.
Orange requires pulling the intermediate servo on the out side of the case.
Red requires a complete teardown of the transmission.
Let's start with pulling the pan. There will be very fine metal powder in the bottom of the pan. This is normal wear, if there are bigger grains of metal or any large peices like clutch material it needs to be rebuilt. You don't need to go further.
If the pan crud looks ok then check for TV cable, bellcrank and valve operation. Make sure everything works smoothly and that the valves in the valvebody all move full travel and the springs are doing what they should.
If everything with the TV cable looks ok i would put a few bolts back in the pan, jack up the transmission and remove the crossmember, remove the distributor cap so you do not break it and lower the transmission down as far as it will go so you can remove the intermediate servo cover. Once you take this apart you will need new seals for it. Numbers 17,18,20 and 22 in the diagram. Inspect the seals before you remove them for any damage so you can see if you found the source of the problem.
Look closely at the orientation of the lip seals so you can install the new seals properly. Lubricate with Vaseline and reassemble.
If you found a damaged seal reassemble and try it out. If no obvious signs of damage were found put the transmission back on the crossmember and remove the pan and remove the valvebody. Check the torque on the center support, use a 3/8 torque wrench torque to 18 foot pounds or 215 inch pounds.
Use no sealer on the valve body gasket, clean everything completly there can be no garket material left anywhere. Do not scar or damage the alumnum case sealing surface. Install the check balls using a lot of vaseline to hold them in place and reinstall the valve body.
If none of those are the problem you will need to have the transmission rebuilt.