Today, I'm pretty sure that I was witnessing the expiration of a vacuum pump. This morning, it was making it's usual 22.5" of vacuum on the way to my garage. After I left (long day) I headed north to the closest town to pick up some cat flea treatment since I picked up another cat Friday. I don't remember looking at the vacuum gauge on the way to town. As I was leaving, I noticed that the gauge was reading 16.5". As I kept driving east for about 20 miles, the gauge had slowly fallen to 14.5". That was all on a speed limit 65 highway. After I used the brakes to turn onto the speed limit 55 road, it was reading 10". As the RPM went back up, the gauge went back to 14" slowly. There's two sets of S curves on that road and, even without touching the brakes, the vacuum dropped to 12" since I kept the truck in 5TH gear to test. As I gained speed and RPM, it came back up to 14". I shut the engine off in my driveway with the gauge reading 6". The needle didn't drop for the 60 seconds (maybe) that I watched with the engine off, so I'm guessing that there's isn't much of a vacuum leak. Tomorrow, I'm going to replace it with the spare that I carry around. It only makes about 12.5" on the highway, but it's pretty consistent about it.