Vehicle window cleaning unit

The windscreen wiper assembly has a telescopically adjustable actuating device with a piston within which a valve seal is retained by a cover (13). The valve seal by its end furthest from the cover seals the piston rod channel (10) in its neutral position in relation to a valve chamber (12) encompassing the seal. The piston has on its circumference at least one through-passage (32) to the valve chamber between the annular seal between the piston and housing, and the cover.