There are mechanical systems in which the most important failures originate from excessive wear. Wear failures gradually develop and can be rather easily monitored or inspected. Design of such systems usually enables one or more adjustments to he performed before a failure requiring replacement of friction elements takes place. It means that such a unit has two kinds of failures: one requiring adjustment and another requiring replacement. When preventive maintenance of a friction mechanism comprising several elements deteriorating by wear, each characterized by a specific wear intensity, i.e. by different failure rates, is considered it is a problem how to determine periods of preventive maintenance so as to provide the best satisfaction of a defined criterion, i.e. the best outputs. The paper presents an approach to the problem solution when the availability or the operational readiness are used as optimization criterion . The model is developed for braking systems of motor vehicles used in large fleets and having different brakes in the front and the rear wheels.