I want to preface my issue by stating that I do not have any brake problems while driving. The peculiar thing is that when I turn the car off and keep my foot on the brake pedal it actually inflates and raises up on its own and gets hard automatically. It is as if I had pumped it. Sometimes while driving when I break it makes like an air sound slightly but that is it. Could this simply be that I have air in my lines which needed to be bled out? What else could it be? Please help.

 

your vacuum brake booster probably has a small leak. so when the truck is running it supplies vacuum to it to keep it working but once you shut it off it probably seeps out pretty quickly and the assist just gives up

check the line going to the brake booster also to make sure its not split or loose