A flow control device for use with flowing fluids to monitor pressure during blood pressure measurements having at least one passage extending from an inlet to an outlet. This passage has two flow paths, one a non-closable, narrow flow path including a capillary bore for normal flow. The other is a flushing chamber having a larger crossectional opening which is closable by a rapid flush valve. The rapid flush valve includes a valve body made of elastically deformable material and disposed in the flushing chamber. The valve body may be stretched by a plunger which thereby reduces its crossection. In the normal position, the outside of the valve body contacts the wall of the flushing chamber and blocks the flow path through the chamber. If the plunger is depressed, the valve body is deformed elastically, elongating it to thereby reduce its crossection and spacing it from the walls of the flushing chamber. This opens the flow path through the flushing chamber.