Waiting on the power steering to pressurize. This is where the power assist comes from on most of these trucks. There is an accumulator on the side of the hydroboost. When you shut the truck off you should have 2 or 3 pushes of the pedal with assist. Sounds like it's bleeding off after the truck sits awhile. No big deal. Just make sure after you shut it off you have a couple of pushes with assist. If not, it could be a problem if the truck ever dies going down the road.