[JOE RILLA]

Actually there is a parking brake "shoe" on each side of the truck. To check it you need to remove the rotors and you will see a horseshoe shaped brake shoe. Most likely the friction material has disintegrated away. this seems to be a common problem at my shop. The parking brake doesn't wear out it just dries up and the friction material falls off leaving nothing for the brake to grab the inside of the rotor with, hence your rolling problem. The other flaw in the design is that the spreader only will push the park shoe out a little bit, so if the shoe is worn or missing part of the friction material it will not expand enough to lock the brake sufficiently.