Complex sentence: The larger west building at Pennsylvania and St. Clair, building C, is the national headquarters, which deals with the mail, archives, and other internal functions of the Legion; the lobbying efforts of the Legion are based in its Washington, D.C. office.
Split into simpler sentences: The larger west building at Pennsylvania and St. Clair, building C, serves as the Legion's national headquarters. The headquarters deals with the mail, archives, and other internal administrative functions of the Legion; the lobbying efforts of the Legion are based in its Washington, D.C. office.