Worked on my car this weekend and managed to make the transmission smoother. Here's how:
1. Got a vacuum check/hand-operated pump.
2. Checked vacuum holding in lines. One was the transmission modulator line (small all black line coming up from the driver's side at the rear of the engine) and it held vacuum perfectly.
3. Checked all the vacuum tube connections I could see. One rubber splitter was particularly bad, some oil on the tubes (I'll check that problem later...), so I used tiewraps on all the lines coming into it. The trans modulator line was plugged into it, so I guess that was a big part of the problem.
Ran the car and wow! It did make a difference. NOTE: as usual, it worked on mine, may not work on all...
Hoping this helps,