gsdan

i have a 1970 buick gs455 with very delayed 1-2 upshift , and no automatic downshift when coming to a stop. I have pulled the governor and checked the weights and the valve inside , all ok , I have pulled the modulator and checked with vacuum and it holds vacuum, and i can see the valve moving . i checked the modulator valve in the trans , and it moves freely. I put the car on jack stands , applied 15" of vacuum to the modulator , and still have the problem . I pulled the valve body , removed all the valves , cleaned and rerassembled , problem still there. Checked the presure valve in front pump , it moves freely as well although I didn't take it out due to not having long enough snap ring pliers. It does have somewhat of a pronounced thump when putting car into gear at idle -600 rpm in drive , 900 in park . I haven't pit a pressure gauge on the car becuase once the problem is fixed , the car will be sold . The trans has a transgo shift kit , and went through all the modifications i did to it to make sure i didn't do anything wrong , and all seems to be in order there. So before I take this car to a transmission shop , does anyone have any suggestions. Thanks in advance