My Automatic Transmission Isn't That Automatic

[thorvintage]

My automatic transmission isnt shifting as well as it should be. I am having to manually shift pretty often. All the gears seem to work well once its in that gear it just doesnt seem to be shifting at the right times on its own.

Also as I am going down the road when it is in 3rd it often goes to neutral until I grab the gear shift and move it to neutral and then back to drive and it reengages until the next time. Starting to feel like have a stick shift without a clutch LOL

Back story: just bought the truck and the guy I bought it from owned it for only a couple months (he had a stroke) put a new used transmission in it and he said he thinks the seals may be sticking a bit or the vacuum modulator may need to be cleaned or replaced. The truck was sitting for almost 15 years before he bought it.

Does anyone have an info or links/videos on how to check, service, or replace the vacuum modulator? I am also open for any other ideas or advise on what I could do to solve the issue...thanks for reading!