There is a diaphram sensor kinda thing about as big around as a quarter that threads into the truck brake valve block. It tells the truck I got the air brakes set or released and will turn your cruise control off when you push the yellow button in. The rubber diaphram inside will get old and blow thru causing air to flow when you push the button in. Easy fix if thats the problem... Maybe 10 bucks for the part and an hour to replace it.