Bako began his professional career with the Pioneer Baseball League's Billings Mustangs, a rookie-league farm team of the Reds located in Montana. During the 1993 season, Bako amassed a .314 batting average, second-highest on the team that season behind Chris Sexton. Bako walked 22 times, stole 5 bases, and batted in 30 runs, while excelling defensively compared to the other catcher on the team. His fielding percentage was .988, and he posted only four errors that season. He was also named a Pioneer League All-Star.