A method of blow molding a reservoir, such as an automotive fuel tank, may include one or more steps. In one step, the reservoir is at least partially formed to have a first wall and a second wall. The first wall defines a first interior and the second wall defines a second interior that may be open to the first interior. In another step, the second wall is contracted inwardly before its material hardens. After contraction, the volume of the second interior is reduced. The contraction may be performed via a mold tool assembly having one or more moveable tool segments.