Wash, string and cut beans in pieces. Cook in boiling water until tender from 1 to 3 hours. Add salt when nearly done. Drain and reserve 1 cup of the bean water for following sauce. Heat and melt the sugar in a dry, hot frying pan, stir in the flour and mix well, then add the hot bean water, vinegar salt and Pepper to taste. When sauce is smooth add the boiled bean and let all come to a boil. Serve hot.