I'm looking to buy this winter beater for 1000$. It has a transmission issue. This is how the owner describes it;

As far as the transmission, when the trans shifts into second and third it wants to return to the previous gear for just a second until the rpm and speed raises slightly, if this makes sense. Cottman Transmission rebuilt the transmission and told my father that the problem was with the vacuum system. I remember my father telling me that a mercedes dealership had to clean oil out of a vacuum valve several years ago. This problem does not act like a slip, but more of an adjustment needed. Once you get into third gear, the ride is smooth.

Now, I thought that a vacuum issue of that sort in an 81' would prevent upshifts and that these symptoms sound like something other than a failed vacuum modulater or simple leak. What do you think is wrong with it?

Thank you.