SUIT CIRCUIT RETURN VALVE should be one-way

Currently if you start with a pressurized suit circuit and a cabin vented to space, pulling the SUIT CIRCUIT RETURN VALVE lever will allow air to leak out of the suit circuit and into the cabin. In the actual spacecraft there are two series check valves to prevent this escape of gas and to only allow the return valve to draw cabin air into the suit circuit when the cabin pressure is slightly higher than the suit compressor inlet pressure.

I don't think this one is a quick fix (and it only affects users who aren't following the proper checklists), but it would be nice to have a realistic implementation of the return valve. Perhaps the flow control levers on panels 300/301/302 could be used to control airflow out of the suit circuit into the cabin when simulating shirt-sleeve vs. suited operations.

Also the suit circuit's O2 demand regulator does not function when the return valve is open. In the initial scenario I described, the O2 flow meter stays at 0 as the suit circuit leaks into the vented cabin. It should be trying to maintain a suit pressure near 3.7 psia when the cabin is depressurized, regardless of the position of the return valve.