Your brake warning light is on because the shuttle valve in your combination valve assembly has moved due to unequal pressure in the front brakes vs. the rear brakes. I struggled with this issue multiple times in the past until I got one of these tools that holds the shuttle valve centered while bleeding the brakes. It screws in in place of the warning light switch.
https://www.amazon.com/gp/aw/d/B07BY...b_b_asin_image
Once the shuttle valve has moved, it will block flow to the rear brakes. This makes it nearly impossible to bleed the rear brakes. You will need to get the shuttle valve centered again. I have read that stomping on the brake pedal can center the valve, but I have never tried this. Bleeding one of the front brakes might allow the shuttle valve to center.
In my experience, the rear brakes are much more difficult to bleed than the front brakes. You know you are done when you get a powerful flow of fluid out of each rear brake when the pedal is being pushed and you crack open the bleeder. There should also be zero air in the fluid that comes out. If the fluid flow is lethargic, then you aren't done.