Diesel Bombers - cruise control off, no engine brake

I have an issue with cold weather only, besides the cold weather. After the cruise control is turned off the idle speed is too fast for the pac engine brake to operate. Brake is turned on by the ECM about 3 seconds after hitting idle speed. Problem is...in colder weather and after cruise is turned off by braking or canceling on the thum control the idle doesent go down enough for the ECM to trigger the brake. If I stop and shut down the engine everything is normal until I use the cruise again. Of coarse the pac brake doesn't work when the cruise is engaged, never has and probably shoulden't. Warm weather... no problem. Turn the cruise off the brake works. This has been happening for a couple years. Never have figured it out. Or had it troubleshot by the dealer, just don't deal with those guys and way too far from town anyway. thanks again for the help, I really appreciate it.