Why does the brake dash warning light come on when engine is idling and go out when engine speeds up.

It could be something as simple as vacuum loss to the brake booster. When you are driving you are creating more vacuum. When you are at idle, you create less. If there is a small leak in a vacuum line, it will be noticeable when at idle and negligible when driving at speed.

you never have to add brake fluid to your car, if the fluid is low in meaning that you need new brakes pads or shoes, or you have a leak in the system, so verify it with your mechanic...