Since you state that the problem started with a VB change, it's possible that the VB maker installed a roostercomb with the incorrect stop plate. The stop plate is cover plate for some of the valves, besides being a cover it provides a stop for the ear on one side of the roostercomb. (area of plate in orange) The pic below shows the two different styles of roostercomb and the stop plate that is correct for each one. If the plate on the right is installed with the roostercomb on the left, it could prevent the roostercomb from traveling all the way to the Park position.