Traditional qualitative modeling techniques have focused on means for abstracting the value spaces of variables that are used to represent system states and for simplifying the constraints that hold among those variable values. In this paper we present a technique for structural abstraction applicable to the domain of pressurized hydraulic systems. Valves, when closed, functionally isolate clusters of components. A cluster can only be in one of two qualitative states -- static, where pressures arc equal throughout and no flow occurs, or dynamic, where flow from a high pressure source to a low pressure sink occurs. Reasoning in terms of clusters is shown to facilitate planning for and explaining about the operation and repair of hydraulic systems.