Abstract: A valve which opens on reduced pressure comprises: a hollow body (1) divided into first and second portions (2 and 3) by an internal partition (4), with the first portion being provided with a first opening (5) for connection to an enclosure to be evacuated and with a second opening (6) for connection to the aspiration side of a vacuum pump; a moving valve plate (7) disposed inside the first portion; a spring (8) urging the valve plate to close the first opening; a rod (10) having one end connected to the valve plate and passing through the internal partition; and a piston (b 11) which is connected to the other end of the rod, and which divides the second portion into a first chamber (13) and a second chamber (14) which are sealed from each other, the first chamber being on the rod side of the piston and being in communication with the outside (19), and the second chamber including an orifice (22) for communication with the outside, which orifice is fitted with closure means (23), the second chamber further i