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Redundant flight control systems, designed in order to ensure the necessary safety level even in failure conditions, may behave improperly during normal operations, if the system architecture is unsuitable, when manufacturing defects are present. This work analyses the response of a secondary flight control hydraulic servo-mechanism, designed according to different redundancy architectures, when manufacturing defects are present. These defects may consist of non-symmetrical overlap of the passageways or incorrect null position with no input current (caused by an asymmetric geometry of the feedback spring or of the first-stage spring, resulting from an undesired magnetic field in the first-stage torque motor, etc.). These defects come into the usual quality standards of the components and must be accepted because they are not removable owing to the accuracy of the manufacturing processes. In the opinion of the authors this paper concerns a topic quite neglected, but important in the technical literature.To the best of their knowledge, no specific scientific work in this field is available, except for some industrial technical reports. Since the above-mentioned problems adversely effect the proper operation of the servo-mechanism, it is necessary to introduce some equalization devices able to improve substantially the improper behaviours. By means of an appropriate physical-mathematical model of a typical electrohydraulic servo-mechanism, the work evaluates the effects of manufacturing defects in different cases and the effectiveness of the eventual compensation devices in limiting the improper behaviours.