. . . "The arrangement is preferably such that the brush 2 is able to yield inwardly from its pre-determined lateral position, i.e., in the event of the brush striking or running against an obstacle such as curb, and for this purpose the hydraulic operation of the or each ram 60 is as shown in FIG. 3 in which hydraulic fluid under pressure is supplied by a pump 55 to each valve 50 for controlled operatio" .