Abstract: A tread-brake unit comprises a brake piston and arrangement (101/100), a slack adjuster (200) and a brake shoe (300). The piston (101) is mounted in the cylinder (100) such that in operation of the brake piston and cylinder arrangement the axis (103) of the piston (101) oscillates about the axis (104) of the cylinder (100). The axis (104) of the cylinder (100) is inclined with respect to the axis (201) of the slack adjuster (200) in the plane in which the axis (103) of the piston (100) is oscillatable about the axis (104) of the cylinder (100). The piston rod (105) of the piston (101)is pivotally engaged with one end of a first lever (401) pivotally mounted in the unit and engageable with one end of a second, bell-crank, lever (410) which is pivotally mounted at its elbow in the unit and is pivotally engaged at its other end to a part (280) of the slack adjuster (200) movement of which within the unit effects, through the slack adjuster (200), operation of the brake shoe (300).

Abstract: A slack adjuster has a non-rotatable fast-threaded output member (19) threadedly engaged by a pair of nuts (20, 21), one of the nuts being a first brake-force-transmitting nut (20) clutchable to prevent rotation thereof and the other being a second adjustment nut (21) clutchable to land. Relative axial movement of these nuts relative to the output member (19) is a measure of "excessive" brake clearance and restoration of the normal spacing between these nuts serves to restore brake clearance to "normal". The nuts (20, 21) are housed in a housing (8) which is non-rotatable but axially movable with respect to the output member (19). The first nut (20) is clutchable to a thrust member (26) resiliently loaded with a force which is overcome only by forces of the magnitude of braking forces transmitted through the slack adjuster.