well there is another thing that can happen with a bad brake booster, the one on my car just went, and all though i have checked every single vaccum line ( just so i know ) but when you are driving the car or some times just sitting at a light, there is a hissing or a whisteling from under the dash.. kinda like when you have a air leak in a mattress and its not constant but its enough to drive you insane! but i had a mechanic look at it and he said that thats mostly the problem < as there are anew drums shoes pads calipers master cylender and lines!) the only thing that is reused is the booster!
but thats another thing that can go wrong, the seals just gie and the car is always trying to adjust the vaccum, to compensate for the leak!
any ways just another idea!