Abstract: An automatic slack adjuster for a vehicle brake linkage comprising a housing constituting a lever in the linkage, and the lever is angularly movable about the axis of a shaft to impart to the shaft a brake-applying rotary movement which it transmits to it through a meshing worm and worm wheel assembly. Automatic worm driving means are incorporated for rotating the worm for adjustment of slack in the linkage in response to relative movement between the lever and a relatively stationary part, and the worm driving means incorporates a connection having a degree of lost-motion equivalent to a desired braking clearance, and a spring-loaded two-part clutch disposed between the worm and a slidable pawl connected to the stationary part.