Brooks, 50, made his debut on the Champions Tour earlier this year and is participating in only his sixth tour event. He racked up an eagle, four birdies and two bogeys in the round, finishing 36 holes at 10-under 132 at Glen Oaks Country Club in West Des Moines, Iowa.
Brooks, a seven-time PGA Tour winner and the 1996 PGA Champion, held a two-shot lead coming into the last hole but bogeyed it, seeing his advantage cut in half.
Calcavecchia posted a 5-under 66 Saturday, notching an eagle, five birdies and two bogeys, claiming sole possession of second place. The 13-time PGA Tour champion is also in search of his first senior win after more than year on the tour.
Sharing third at 8-under 134 were Bob Gilder (66) and Peter Senior (67).
Jay Don Blake (64), Brad Bryant (65), Mike Goodes (66), Steve Pate (66) and Rod Spittle (68) were all knotted for fifth at minus-6.