weak breaks 97 6.5 /K1500
 

My breaks suck. very soft pedal, and spongy.
I have bled the system several times starting right rear and working closer. It has a new master , and a new Hydro boost installed, new front calipers, and rear wheel cylinders.

When bleeding it seems the fluid to the rear is not flowing like it should. I get flow when bleeding but not a lot. pedal drop when I open the rear bleeders is slow, unlike when I open the front bleeders . The fronts are instant pedal drop and lots of flow . Pumping the pedal does not seem to bring it up like I would expect it to. I vave bled it to death , used a full quart of fluid, and still no results.
The brake light warning stays on , and with hard breaking the ABS does work like it should.

I am at a loss on what is happening here . I have done many many break jobs before and never had a problem getting good pedal .
Any suggestions on trouble shooting this Hydro Boost system. The sister truck with the same system has great breaks and pedal.