A pressure enclosure includes a first component having an opening, a second component coupled to the first component in a position over the opening, a third component positioned between the first and second components and covering the opening, and a load chamber defined by a space between the second and third components and configured such that pressure in the load chamber biases the third component against the first component to seal the opening. The pressure enclosure may be a cylinder of a pump for pressurizing fluid or gas, with the first component a cylinder body, the second component an end cap and the third component a valve body, with the load chamber biasing the valve body against the cylinder body.