Weeks' shot in the fifth inning handed the Brewers a 4-1 lead.
Marcum (7-4) held the Padres to a pair of runs on six hits, overcoming four walks while striking out two.
San Diego's Clayton Richard (14-14) smacked a solo homer the plate but was roughed up on the mound, surrendering five runs on 10 hits through seven innings.
Yasmani Grandal supplied two hits and an RBI for the Padres.
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