An employee was rotating blankets in the die cut section while a maintenance employee started working on another section of the machine. The employee informed maintenance that an air actuation brake was not working and they came to assess the issue. Maintenance employees pressed the brake switch but it did not engage. As the employees were troubleshooting, the injured employee walked to the drive side of that section and pointed to the piston that was not engaging. The piston brake engaged unexpectedly onto the injured employee's hand, amputating the first digit of his left index finger.