EE WAS REPLACING THE PARKING BRAKE PADS ON EQUIPMENT. THE BRAKE ASSEMBLY WAS REMOVED & PLACED ON A WORK TABLE TO REMOVE THE PADS. EE WAS UNAWARE THAT THE CAGING STUD WAS ALMOST OUT (UNSCREWED) OF THE PISTON, ONLY 2 THREADS HELD THE STUD IN THE PISTON. EE COULD NOT SEE THE PISTON MOVING BACK AS IT SHOULD. AS A RESULT, HE PLACED HIS LTINDEX FINGER INSIDE. PISTON CLAMPED CLOSED.