Here's a link to interpreting vacuum gauge readings. I have a vacuum gauge in my '88 Escort also tee'd into the brake booster line. It runs around 20-21 in. vacuum at idle. Does the vacuum go above 15 in. on deceleration? It should. Mine goes to about 24-25 in. vacuum on deceleration. 15 is lower than I'd expect, but it will vary depending on your elevation. Vacuum pressure will drop about 1 in. per each 1000 ft. above sea level. A few years ago I changed the timing belt on the Escort and the vacuum dropped to around 16-17 in. vacuum at idle until the timing belt wore in a stretched some so maybe the valve timing was slightly off after the belt change?? Never quite figured out exactly why the change in vacuum after the belt was changed.