A self-setting automatic slack adjuster assembly for a S-cam brake for a vehicle comprising with the following: (a) a body connectable to a brake actuator by clevis pin at bush at its one end and housing a worm wheel having internal splines corresponding cooperatively with the S-Cam shaft; (b) a clutch mechanism wherein a clutch worm gear forms a clutch with rotatable worm shaft by mutually engageable serrations (4" - 6'), the said worm shaft meshing with the worm wheel but perpendicular to its axis, the said worm shaft retained against a heavy compression spring preset to a determined load (c) control arm assembly including a rotatable control arm with a fixing portion secured to the vehicle chassis to provide a reference point and a control gear wheel with circumferential teeth also disposed in the bore of the worm wheel but integral with control arm and (d) a pinion assembly cooperating with the control gear wheel and comprising one way lock mechanism, and a multi start worm screw engaging the clutch worm gear and also incorporating a gap sensing arrangement to achieve desired slack, wherein: the body, the clutch mechanism, the control arm, and the pinion assembly are all rotatable relative to each other; characterised in that if: (i) the gear ratio between the clutch worm gear and the multi-start worm screw is Z: 1 where Z is a numeral, and (ii) the angle between two adjacent teeth of the one way lock is X degrees, where X is a numeral, and (iii) the angle between serrations on worm shaft and serrations on clutch worm gear is degrees, where Y is a numeral, then: the value of X and Y is so selected that the value of X/Z is less than the value of Y, such that automatic adjustment of the slack is achieved by control of the one way lock only. (a) a body connectable to a brake actuator by clevis pin at bush at its one end and housing a worm wheel having internal splines corresponding cooperatively with the S-Cam shaft; (b) a clutch mechanism wherein a clutch worm gear forms a clutch with rotatable worm shaft by mutually engageable serrations (4" - 6'), the said worm shaft meshing with the worm wheel but perpendicular to its axis, the said worm shaft retained against a heavy compression spring preset to a determined load (c) control arm assembly including a rotatable control arm with a fixing portion secured to the vehicle chassis to provide a reference point and a control gear wheel with circumferential teeth also disposed in the bore of the worm wheel but integral with control arm and (d) a pinion assembly cooperating with the control gear wheel and comprising one way lock mechanism, and a multi start worm screw engaging the clutch worm gear and also incorporating a gap sensing arrangement to achieve desired slack, the body, the clutch mechanism, the control arm, and the pinion assembly are all rotatable relative to each other; characterised in that if: (i) the gear ratio between the clutch worm gear and the multi-start worm screw is Z: 1 where Z is a numeral, and (ii) the angle between two adjacent teeth of the one way lock is X degrees, where X is a numeral, and (iii) the angle between serrations on worm shaft and serrations on clutch worm gear is degrees, where Y is a numeral, then: the value of X and Y is so selected that the value of X/Z is less than the value of Y, such that automatic adjustment of the slack is achieved by control of the one way lock only. (i) the gear ratio between the clutch worm gear and the multi-start worm screw is Z: 1 where Z is a numeral, and (ii) the angle between two adjacent teeth of the one way lock is X degrees, where X is a numeral, and (iii) the angle between serrations on worm shaft and serrations on clutch worm gear is degrees, where Y is a numeral,