Very shortly after the outbreak of the First World War, Brück entered military service with the Imperial German Army on 2 August 1914 as a one-year volunteer.

[1] Discharged from the service, Brück attended a mechanical engineering school in Cologne from 3 October 1916 until December when he legally was required to return to work in support of the war effort, taking a clerical job.

From 1920 to 1922, he was a member of the Deutschvölkischer Schutz- und Trutzbund, the largest and most influential anti-Semitic organization in the Weimar Republic, and served on its board of directors.

[2] In September 1927, Brück was appointed Ortsgruppenleiter (Local Group Leader) in Saarbrücken and Deputy Gauleiter of Gau Saar, the first holder of this post.

In 1934 he became an Organizational Director of the united "German Front," an alliance of Saar political parties founded to advocate for unification with Germany in the upcoming plebescite of January 1935.