With the win, the Brewers' seventh in eight home games since getting swept by the Cubs in a four-game series in July, they matched their high-water mark this season at 19 games over .500 and retained a 2 1/2-game lead on the Cardinals in the National League Wild Card standings.
"Good teams put the pedal to the metal, especially at this time of year," Brewers manager Ned Yost said.
Milwaukee pulled to within 4 1/2 games of first-place Chicago, which lost at home to the last-place Washington Nationals.
Bush & Co. suffered no such letdown against the Pirates, who are in fifth place in the NL Central and sit 21 games out of first place. Bush won his third straight decision with seven innings of work, and Mike Cameron, J.J. Hardy and Corey Hart each drove in two runs to beat left-hander Zach Duke (4-12) in front of the 19th consecutive sellout crowd at Miller Park.
"Last year at this time, we were kind of tanking," Bush said. "This is definitely a lot more fun."
Bush, who was demoted to Triple-A earlier this season before Yovani Gallardo's knee injury forced the Brewers to bring him back, has been having a lot of fun of late. He improved to 3-0 with a 2.30 ERA in his four August starts and is 4-1 with a 2.93 ERA in eight starts since the start of July.
He's also 6-3 with a 3.21 ERA in 13 starts this season at Miller Park.
The Pirates jumped to an early lead. Nyjer Morgan led off the game with a single, stole second, then advanced to third and scored on a Ryan Doumit groundout. In the second, Bush retired the first two hitters before Jack Wilson doubled. Wilson scored when Duke poked an 0-and-2 pitch to center field for an RBI single and a 2-0 lead.
Bush then retired the next nine hitters he faced. In his seven innings, he allowed three runs on seven hits with one walk and two strikeouts.
"That's been exactly what he's been doing the last eight to 10 starts, give up one or two in the first inning and then get into 'mow-down' mode," Yost said. "Quick innings, low pitch counts, on the attack, executing pitches. He did it again tonight.
"In my mind, it's like, 'Let's get through the first inning and then get rolling and here we go.' I watched Tom Glavine do that for years. If you didn't get him in the first inning or the second inning, you were done. Bush has really got his season turned around."
Hart tied the game at 2 with a two-run double in a three-run fourth inning, aided by two Duke walks and one of the Pirates' two errors in the game. Bill Hall put the Brewers on top with a sacrifice fly in the fourth and Hardy tacked on a key solo homer in the fifth to send Duke to his eighth consecutive loss.
Duke was charged with four runs on six hits in six innings.
"He's got good stuff; maybe he's unlucky," Hardy said. "We don't miss mistakes when he's pitching."
Hardy's 20th home run this season gave the Brewers a 4-2 advantage and loomed large when Bush surrendered a solo homer to Pirates outfielder Brandon Moss leading off the seventh.
Bush escaped the inning without further damage, and won after allowing three runs on seven hits in seven innings. He has pitched at least into the seventh in each of his past four starts and in seven of his past eight starts.
He said the April demotion had nothing to do with his midseason turnaround.
"Honestly, I'm not much for motivating myself like that," Bush said. "I try to take the emotion out of it. I feel like I pitch better if I can try to stay calm and under control and focus on what I am doing. There's no day when I woke up and felt better than I had been."
Milwaukee opened up a 10-3 lead with six runs in the seventh inning against relievers Denny Bautista and Sean Burnett, who surrendered a two-run home run to Cameron that glanced off the right-field foul pole. Pinch-hitter Laynce Nix replaced Bush and drew a walk leading off the frame, and was on deck waiting for his second plate appearance of the inning when Jason Kendall grounded into a double play.
Along the way, Brewers first baseman Prince Fielder picked up a bonus RBI. Left fielder Ryan Braun, who returned to the lineup with two hits including an RBI double and two runs scored, stole third base before Fielder hit a sharp grounder to second base. It was an easy out for Freddy Sanchez, but Sanchez took too long to throw to first for the out, and Braun scampered home.
"That's what you get from a baseball player, a guy who has built-in baseball instincts," Yost said of Braun.