Abstract: An annular reservoir is defined by two mutually rotatable annular housing portions having suitable seals and friction reducing members that the housing portions can maintain fluid pressure within the reservoir despite relative rotation between them. Each housing portion is suitably ported for fluid transmission between the reservoir and an external point, such that one member may be connected to an external supply of fluid under pressure and to pressure controls, and the other member may be connected to a fluid powered mechanism such as an hydraulic cylinder, adaptable to perform a mechanical function such as closing or opening a valve. One housing portion may be channel shaped and the other may be receivable within the channel. A side of the channel shaped housing portion is removable to permit assembly and disassembly of the actuator.