The Brewers were clinging to a 1-0 lead heading into the big frame and came out with a 5-0 advantage thanks to the trio of long balls.
Maldonado finished 3-for-4 with three RBI for Milwaukee, which took the opener of a three-game series.
Yovani Gallardo (10-8) won his second straight start, limiting the Reds to one run on six hits over seven frames. He overcame three walks and struck out four.
Bronson Arroyo (7-7) was reached for five runs on 10 hits in a 5 1/3-inning stint for the National League Central-leading Reds.