1998 Chevy Z71 Engine question

Hello,
I have a 1998 Chevy Z71. I have always noticed that when going down the highway and I let off the accelerator, it takes a second for the engine to
idle down and begin slowing the truck. (I was told this was common). This "let down" had a vacuum feel to it.
The problem is that for the last year or so it doesn't want to idle down, i.e., I have to apply the brakes to get the engine to start "letting down" and then the engine doesn't help slowing the truck much. Obviously this is a pain when going downhill or exiting on a downhill offramp.
Any help is much appreciated.
Travis Benedict
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My 99' does the same thing. When I change the oil I spray fuel injection cleaner around the throttle shafts of the fuel injection unit. I also clean the throttle plate at that time. Seams to do the trick. But there has always been a slight hesitation to the idle down.
Brian
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