Even though the injectors are under the valve covers, where you would think that they would be protected from the elements, the injector lines typically develop corrosion where they seal to the injector. The problem is that water makes its way down the line from outside the valve cover and has nowhere to go. The line then rusts all around the injector. The sealing surface does not rust, but when you take the line off there is a slim chance that the next time you tighten it down it will not seal in the exact same spot. Plus, you have a change of getting debris into the injector if the line is not cleaned. Huckstorf Diesel recommends replacing the lines any time corrosion exists.