A gravity reactive braking system comprises a housing connected to a structural element of a manually powered wheeled unit； a guide wheel having a first sector of radially protruding elements positionable in drivable contact with a main wheel periphery, and a second sector with a circular periphery of a shorter radial length than the first sector； a plate configured with a cammed surface； a height adjustment unit； and a follower element connected to a brake assembly cable and pivotably mounted onto the housing, for movably engaging a plate periphery. The guide wheel is rotatable in a rotational direction opposite to the main wheel's rotational direction when the radially protruding elements are positioned in drivable contact with the main wheel periphery following suitable manipulation of the height adjustment unit, and is indexable a predetermined angle until a desired region of the guide wheel is in contact with the main wheel periphery.
