Many organizations rely on the effective use of fleets of repairable equipment that must perform sets of sequential missions. Such fleets include production equipment used in manufacturing systems, material handling equipment used in warehousing, and transportation vehicles used in distribution. Unfortunately, most organizations do not possess sufficient maintenance resources to keep the fleet in top condition at all times. Therefore, maintenance managers must make decisions regarding the allocation of the available maintenance resources. This task falls within the domain of selective maintenance, the process of identifying the subset of maintenance actions to perform from a set of desirable maintenance actions. In this paper, we consider a selective maintenance model that is used to maximize fleet reliability, the probability that all systems within a fleet successfully complete their next mission. We evaluate fleet reliability over sequential sets of missions where system maintenance is performed only during the breaks between mission's sets. A combined simulation-optimization model is used to simulate the performance of the fleet over each set of missions and identify the optimal maintenance activities to perform during each break. We use this model to analyze the behavior of the stochastic process corresponding to the number of successful missions in each set.