Abstract: A checkvalve (1) is provided with an inlet which can be connected to an operating system requiring a negative pressure, e.g. to a braking force amplifier (17), an outlet (3) which can be connected to a negative-pressure system producing a negative pressure, a main air channel (4) connecting the inlet and the outlet in a fluid manner and enabling an air flow in the direction of the outlet, a first checkvalve (9) installed in the main air channel (4), a single outside-air channel (5) which branches off downstream of the first checkvalve (9) and lets out into the atmosphere and a cross-sectional narrowing (7) in the manner of a venturi pipe (6), a venturi channel (8) connecting the cross-sectional narrowing (7) upstream of the first checkvalve (9) to the main air channel (4) and a second valve (10) located between the inlet (2) and the cross-sectional narrowing (7) in the main air channel (4) or in the venturi channel (8).