Well you have a vacuum leak, but just where is it coming from. One of two ways that this is happening to you. One and I hope this is where it's at for you sake would be the check valves that are in the vacuum lines under the hood, going to the central hookup over on the left side behind the engine.
The second area to look is the vacuum motors that are under the dash. If you can't replace these yourself it's going to get expensive. If it's the servo thats on the driverside with two vac line running to it (which is the most common) you can find which of the lines will not hold vacuum and disconect that line and plug the vac line going in and make do with it, or go for the 4 hour job of fixing it.
Hope that helps
Poolman