able to separate the housing 15 into an upper primary chamber 165 or first primary chamber and a lower primary chamber 170 or second primary chamber the piston 20 is translatable relative to the housing 15 along the direction z between an upper neutral position and a lower neutral position the piston 20 is translatable along the primary direction z the piston 20 is made from a metal material preferably aluminum the piston 20 includes a peripheral receiving groove 180 for receiving a piston seal 175 a passage opening 185 for the stem 30 and sealing means 190 for the opening 185 the piston 20 is fastened to the shaft 25 using a screw 252 engaged in an axial tapping 254 of the shaft 25 the screw 252 traverses a screw orifice 202 arranged at the center of the piston and centered on the axis a 4 two washers 256 and 258 respectively positioned in the first primary chamber 165 and in the second primary chamber 170 are axially tightened around the orifice 202 by the screw 252 and the shaft 25 one end of the shaft 25 opposite the piston 20 is coupled to the pumping stage 8 the shaft 25 is cylindrical and centered on a fifth axis a 5 combined with the first axis a 1 the shaft 25 is received in the primary bearing 160 the shaft 25 is translatable with the piston 20 along the primary direction z the reversing stem 30 has a first end part 192 a central part 193 and a.