Methods and devices for preventing temperature and pressure extrusion failure in o-ring seal assemblies. One embodiment employs a bore in the plug so that the thin wall of the plug flexes outward at high pressures to partially fill the gap between the chamber member and the plug where the o-ring is located, thereby preventing the o-ring from extruding through the narrowed gap. A second embodiment utilizes a plug of a higher coefficient of thermal expansion than the chamber member that expands outward slightly at high temperatures, thus partially filling the gap. A third embodiment secures the o-ring in the gap between a backup ring and a retaining ring, and the backup ring has a higher coefficient of thermal expansion than the plug and chamber member. At high temperatures the backup ring expands to partially fill the gap. Additional embodiments are created by combining these three embodiments.