I think you did a good job of explaining it...There are contacts under your brake levers. When you pull them, they separate and the brake light is energized.
The running light will be dimmer than the brake light , esp when idling. When at speed, it will get more current and be brighter.
The brightness of the brake element should exceed the running light, which makes sense, as you want it to be seen.
I think with the proper 6v bulb the running light may be stronger at idle, but it is more convenient getting the 1157 bulb.