the pressure stages on the course of the cycle in addition it makes it possible to reduce the height of the valves which results in more space being obtained the spatial requirement being reduced and obtaining a reduction of weight according to another characteristic of the invention the housing of the valves fig4 is situated in a small volume annular chamber fig5 the transfer ports 7 fig5 forming part of this chamber this preferred disposition enables the valves to react at the slightest partial vacuum in a minimum period of time the closing plate covers the seat at the right of the opening of the chamber over a predetermined distance ensuring the imperviousness of the valve according to a third characteristic of the invention the shape of the valves is partially cylindrical at the level of the closing plate 25 fig4 of the orifice of the annular chamber this shape renders the valves highly rigid which enables them to be manufactured with a technical plastic because of this the weight of the valve is extremely light 0 5 to 1 grams this light weight makes it more mobile as it can beat over 100 times per second according to a fourth characteristic of the.