Abstract: A fluid reservoir and pressure protection device for use with fluid-filled systems comprises a cylindrical chamber wall, bottom chamber wall and depending mounting shank defining an inlet opening. A first piston slides in the cylindrical chamber wall between upper and lower positions, and has an annular working surface defining a central opening. A hollow cylindrical piston stem extends upwardly from the central opening. The first piston is spring biased downwardly. A second piston slides in the hollow piston stem between upper and lower positions. The working surface of the second piston, the annular surface of the first piston and the hollow piston stem together close a variable volume chamber. The second piston is also biased downwardly.