Brake pedal low

Are you saying you tried to bleed the ABS hydraulic controller by forcing an ABS stop while driving? Where is the air supposed to go? You need a scanner to command the ABS computer to hold two of the valves open so the air can be expelled and be pushed down to the wheels. That only works while the bleeder screws are open. These guides can help you fix it.

https://youtu.be/w7gUsj2us0U

and

https://youtu.be/WDxvEQrMkBg

and

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Please run down this guide and report back.

My concern though is the "slowly sinking" part of your description. That would apply to a master cylinder that was damaged by pedal-bleeding and pushing the brake pedal all the way to the floor, (does not apply to a master cylinder less than about a year old), or a leaking brake line or a badly-leaking wheel cylinder. It is also not uncommon to see what appears to be a slowly-sinking pedal from gradually increasing foot pressure on the brake pedal without realizing it. I have done that many times.

A valve on the rear axle will be a height-sensing proportioning valve. If I am right, there will be a link between it and the frame. That only changes the rear brake fluid pressure limit for changes in loading so you get more rear brake power when carrying a heavy load.

If we were to add your observation of delayed release of the brakes to the slowly-sinking pedal, I would be looking for a constricted rubber flex hose, typically a front one, but I cannot recall ever running into that on a GM truck. Usually we get those when they are so bad, a front brake does not release at all. With a partially blocked hose, you will not be able to get fast enough flow during bleeding to get the air to go down to the wheel. It will continually float back up as the brake fluid goes down. This is the one time I would use a helper to push the brake pedal. Have him push the pedal half way down, fairly quickly, while you watch at each wheel for a nice rapid flow of fluid from the bleeder screw. If you just see a little trickle, see how that compares to loosening the steel line at the master cylinder. You will get good flow there if it is the flex hose that is restricted.