Exemplary embodiments of a gas biased pressure regulator comprise a capsule subassembly and a regulator body. The regulator body is adapted to connect to a source of pressurized gas. The capsule subassembly is removably received within the regulator body and includes a capsule body and a piston. The piston is axially slidable between fluid release and fluid seal configurations. When the piston is in its fluid release configuration, an output chamber is placed in fluid communication with the source. When the piston is in its fluid seal configuration, the output chamber is sealed from fluid communication with the source. A pressurizable bias chamber within the capsule body contains a bias pressure urging the piston toward its fluid release configuration. Pressure from the source urges the piston toward its fluid seal configuration. The piston may also have a fluid charge configuration for facilitating the pressurization of the bias chamber.