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In page Bruno Sammartino:

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In early 1977, Sammartino informed McMahon Sr. that he felt he could no longer continue as champion due to his injuries. On April 30, 1977, he was defeated by Superstar Billy Graham for the title.[2][6] His second title run lasted three years, four months, and twenty days (1,237 days).[4][7] Despite a very long series of rematches against Graham, Sammartino was unable to regain the title. His final attempt was in Philadelphia, just a few days before Graham was scheduled to lose the title to Bob Backlund.