I am driving along with the cruise control set and the air conditioning on. All of a sudden I notice that I am slowing down and that the air conditioning is no longer cooling the car but air is coming out of the heat vents. It is a vacuum leak somewhere in the system.
Vacuum supplies a lot of functionality in the car.
Normally there is also a performance problem associated with a true vacuum leak. There is a major vacuum leak associated with a wide open throttle and that is the throttle body itself. When you step into it, put the pedal to the metal, floor it, whatever you call it, the vacuum in the intake manifold, the source of vacuum for all that stuff I mentioned above, approaches atmospheric pressure, in other words, there is no longer a source of vacuum to supply these functions. Well the engineers thought about that problem and therefore most modern cars have a vacuum accumulator under the hood somewhere. Since a vacuum is really the absence of anything this accumulator collects . . .