Abstract: An automatic slack adjuster has an adjustment mechanism which includes a ratchet wheel having ratchet teeth and being rotatably mounted to a lower portion of a pivotal arm of the adjuster and drivingly coupled to a set of gears for causing brake shoe and drum clearance adjusting rotation of the gears upon rotation of the ratchet wheel. The mechanism also includes an adjusting assembly having ratchet teeth complementary to the ratchet teeth of the ratchet wheel and a pair of links pivotally mounting the adjusting assembly to the arm and a push rod pivotally connected to the arm to define a pivotal parallelogram linkage disposing the adjusting assembly with its ratchet teeth in a one-way driving relation with the ratchet teeth of the ratchet wheel.