INJURED & 2 OTHER EMPLOYEES WERE DISMOUNTING AN 1800 X 33 TIRE FROM FRONT AXLE OF TRUCK. HE INSTALLED HYDRAULIC BEAD BREAKER ON THE TRUCK SIDE OF THE WHEEL TO PUSH TIRE OFF OF RIM. WHEN PRESSURE WAS APPLIED TO BREAKER IT CAME OFF THE WHEEL. HIS HAND WAS ON THE BREAKER WHEN IT CAME LOOSE AND WAS MOMENTARILY STRUCK BETWEEN THE TRUCK FRAME AND THE BREAKER.