The GSX profile supplied by FlybyWire has set a custom Parking Brake expression which I assume it's correct, also because I tried it shortly, and it works here. The custom expression relies on two custom LVars to be set so, the possible reasons for them to not be set might be, considering your entire cockpit is black:
- The airplane is in Cold & Dark state, and those variables are not set in that state.
- The airplane code is not even running, also explaining why the variables are not set.