Abstract: A brake system tool for measuring the slack adjustment between an air cylinder and the brake shoe engagement with the drum of a conventional braking system. The tool includes an elongated tubular body carrying a bushing which slidable supports an indicator. The tool or slack adjustment tester as it is commonly called is held in place by a base releasably attachable to the air cylinder of an air brake system. The base is connected to the indicator. A sliding pin body is adjustably held on the tubular body by a set screw and carries a removeable pin which engages a slack adjuster arm or equivalent throughout its movement. The movement is displayed on the indicator as the tubular body is moved uncovering the indicator for a reading of the indicia.