A surface treating head includes a main body; a brush unit; and a drive mechanism for moving the brush unit relative to the main body between a stowed position and a deployed position. The drive mechanism includes a pressure chamber and a valve unit for varying the pressure within the chamber. The valve unit includes a housing, a valve located within the housing and, disposed on the housing, an actuator moveable relative to the housing through engagement with a surface to be treated for operating the valve and at least one surface engaging member extending downwardly beyond the actuator. The housing is moveable relative to the main body to maintain the surface engaging member in contact with the surface to be treated.