An employee had a new piggyback brake chamber on the work bench and needed it caged. The employee was using a blow gun to simulate the brakes releasing so he could install the caging bolt. While installing the caging bolt by holding/squeezing the bolt, the blow gun lost its seal on the fitting, which caused the threads of the bolt to slide across his left fingers. The employee lost flesh from his middle finger, and to a much lesser extent his index finger.