Abstract: A tool for measuring and adjusting slack movement related to operating components of an air-brake assembly has an elongated lever portion for enabling leverage force to be applied to a rocker-arm component of the air-brake assembly, an abutment fixture fixedly attached to the first end of the lever portion, the abutment fixture providing a fulcrum point against the rocker-arm component for application of leverage force, and an engaging member fixedly attached to the first end of the lever portion at a point substantially inward from the attachment position of the abutment fixture. The engaging member is for engaging the rocker-arm component on an edge of the rocker-arm component opposite an edge engaged by the abutment fixture. A socket mount is fixedly attached to the second end of the lever portion at a position substantially inward of the outer edge of the second end, the socket comprising a socket for fitted engagement of a slack-adjustment nut of the air brake assembly.