I have an American Standard bathtub valve that keeps leaking, even after replacement of the valve assembly and seat. What would cause this and how can I fix it?

Turn off water and remove stem. Make sure washer is not damaged then reinstall stem. When reinstalling stem make sure that it is in the open position. If the stem is in the closed position during installation, the washer can contact the seat before the stem is fully installed. Then when in use, the washer does not press against the seat with sufficient pressure.