[B. W.]

The dash light is coming on because there is a pressure differential between the front &rear brakes. (unsafe condition). Usually caused by a leak.

The reservoir for the rear brakes is low because there is a leak in the rear brake system - probably the wheel cylinders.

The wheel cylinders leaking =grabby brakes (brake fluid on shoes) & low/empty reservoir.
The reservoir being empty = no pressure = light on.