Marcum (4-3) controlled the Pirates, yielding one run on five hits while not allowing a walk in winning his second straight start. The right-hander has surrendered only two runs over 14 innings during the span.
Conrad provided a sacrifice fly in the second inning and then capped a four-run fourth with a two-run shot, his first homer of the year since being called up from Class AAA Nashville Monday.
Cody Ransom added a two-run single for the Brewers, who evened their three-game series at Miller Park.
Pittsburgh starter Erik Bedard (3-6) was touched for all five runs on five hits in a 3 2/3-inning stint.
Garrett Jones went 2-for-4 with a solo homer for the Bucs.