Complex sentence: Between 1946 and 1957 the Roadmaster served as Buick's flagship, and when it was resurrected for the 1991 through 1996 model years it was the marque's largest vehicle.
Split into simpler sentences: Between 1946 and 1957 the Roadmaster served as Buick's flagship. When it was resurrected for the 1991 through 1996 model years, it became the marque's largest vehicle.