now, when he is born in Graz directly you can state this to the diocesan-archive and you can ask in english. we learn it at school in Austria :). think they will find Johannes Rucker born 16.5.1893 in their Graz-files. tell them also this details > "My father, father's name was Franz and his mom was Helen. Had siblings, but my mom told only one name- Victor".:
http://www.katholische-kirche-steiermark.at/dioezese/ordinar...maybe you tell us, if you got helpful info, so you can make more research to more other ancestors after that. if you know then the parish-church or so.....
there is also possibility, that you have relatives in Graz directly:
http://www.herold.at/telefonbuch/graz-stadt/rucker/or in Graz/Umgebung (surrounding):
http://www.herold.at/telefonbuch/graz-umgebung/rucker/http://www.herold.at/en/telefonbuch/