Brake fluid does not flow to the rear brakes?

First, I would loosen the lime to the master cylinder and then press on the brake pedal to see if fluid comes out of both ports, if not the master is bad. If so, tighten the lines before letting up on the brake pedal. Also, try to remove the brake bleeders because they can become clogged with rust and not allow the fluid to flow. Here are guides to help us see how to bleed the system and how to replace the brake master if needed:

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

and

https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system

You may need to open the valve inside the ABS unit due to air the system.

This video will help you see how that is done:

https://youtu.be/nQRnKyVyMuQ?si=XOxQXSZepir2L9gl

Please upload pictures or videos of the problem so we can see what's going on.