In a development of the invention, the positioning unit is configured in the manner of an armature which is rigidly attached to the piston and is axially displaceable in a specified manner by {HEAD ~ means} of {TAIL ~ a proportional magnet} 
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In a development of the invention , the positioning unit is configured in the manner of an armature which is rigidly attached to the piston and is axially displaceable in a specified manner by { HEAD ~ means } of { TAIL ~ a proportional magnet }
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