the first casing 23 as a result the lower end of the pipe 28 is thrust into the valve assembly 30 and into abutting engagement with the upper face of the movable valve body 33 whereupon the movable valve body 33 is pushed downwardly within the valve assembly 30 against the force of the spring 34 to communicate the inner space of the valve assembly 30 with the inner space of the first casing 23 consequently air is drawn into the inner chamber of the first casing 23 through the narrow passage 31a and the inner space of the valve assembly 30 thereby increasing the pressure or lowering the vacuum in the inner chamber of the first casing 23 and driving the valve assembly 30 downward by the force of the spring 38 this causes the pipe 28 to shift out of the inner space of the valve assembly 30 to close the same with the movable valve body 33 as shown in the drawing whereupon the pressure in the inner chamber of the first casing 23 is lowered i e to a higher vacuum driving the valve assembly 30 upward in this manner the downward and upward displacements of the valve assembly 30 are repeated to maintain the pressure in the inner chamber of the first casing 23 at a constant level in.