Abstract: A master cylinder employed in connection with a fluid actuated motor vehicle clutch system. The master cylinder includes an internal reservoir formed by a portion of the same cylindrical housing which makes up the clutch actuating working chamber. A plug is provided within the cylindrical housing to divide the housing into two sections, a working chamber and a reservoir chamber. The plug further includes a check valve system which permits fluid to flow into and out of the working chamber from the reservoir as the fluid capacity of the associated hydraulic circuit changes. Flow into and out of the reservoir is permitted to occur only when an externally applied load to the master cylinder is released. The reservoir chamber includes a slidable piston and a low spring constant helical spring which biases the reservoir plunger to provide a slight hydrostatic pressure for the fluid contained by the reservoir, thereby preventing the ingestion of air or contaminants.