Sounds like you are talking about the modulator. There is a black rubber cap that when removed covers a key which alters the pressure given at any vacuum level.
Proper adjustment is done with a pressure gauge hooked to the appropriate port. At a certain rpm with the vacuum removed the pressure should be that which is shown in the manual for your transmission.
The actual working of the vacuum modulator is severely dependent on the proper working and adjusting of the vacuum system. This is probably the problem in 99 out of 100 harsh shifting problems. There is no point in trying any of this without a vacuum gauge and a vacuum schematic.
There are a few reasons why there is value in that 125 dollar switch replacement. But, it seems they didn't finish the job. I could tell you how to do it in a few hours. After a year or two, you could do in a few hours of experimenting what I can do in 15 minutes. How much is it worth??
Bosch Master, ASE Master, L1
33 years MB technician