A pressure therapy device includes a pressure chamber, an inflatable padding, a seal, and a positioning mechanism. The pressure chamber has an opening arranged for admitting the inflatable padding and a limb of a user. The inflatable padding is inflatable to enclose and fix the limb in position. The seal covers the opening, including the inflatable padding and the limb to seal the pressure chamber from ambient atmospheric pressure. A pump unit is provided to generate a non-atmospheric pressure within the pressure chamber and includes a first valve system and a piston with safety release features for preventing unsafe pressure levels.