Cannot get brake fluid to back brakes

When you bled the brakes and opened the rear bleeders while holding the brake pedal down, did it drop at all?

If not then you either have a restriction in the system or possibly a master cylinder issue.

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

A hard brake pedal could be just a brake booster issue but if we are not getting a pedal drop when bleeding the system then we need to look at the above issue. Let me know and we can go from there. Thanks

So we need to find the restriction. I would start at the master cylinder and work backwards. Basically you are looking to find where the brakes get hard again. You could have an issue with the proportioning valve or the master cylinder itself.