Someone else recently posted about replacing this $.05 three-four inch rubber connector tube and they said it was like they had a new transmission. If this leaks, you can ATF/filter change, synthetic AFT and adjust cables till the cows come home, but it will not make a real difference.
Check the full run of the plastic tube, from the engine to the trans, the rubber connector (oh BTW even if it *looks* OK, I'd just replace it while you are down there, and then the vacuum modulator valve.
Speaking of which, I had a 300D that I worked on for tons of hours, had several techs fiddle with etc. Finally replaced the vacuum modulator valve and WOW .... back to the way it was suppose to be.
Hope this helps, keep us informed.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)