External Pressure Controlled Valve.

I'm in the process of designing a rebreather for diving and I've run into a snag. I need to regulate the pressure in the "breathing bag" in conjunction with the increase/decrease in pressure on the outside of the "bag". The way I understand it a standard release valve is set by compressing a spring to supply the resistance for the valve. Do you get a commercial valve that could be adjusted by an external source? I know dive regulators have this function but I'd really not have to reinvent the wheel if I can get one off the shelf. I'll be looking at keeping the internal pressure of the "bag" at about 0.1 bar higher then the out side pressure.