90/brake problems

hey i have put all new harware and brake cylinders in the back, replaced one caliper, rotors are fine not warped or cracked, replaced one of those last yr, but here;s the problem, you can go all the way to the floor sometimes, now i have bled them out all the way around and still they dont come and hold sometimes like they use to, no loss of fluid, however, i used to big bottle of fluid while bleeding cuz the old fluid was dingy brown, perhaps i should do a complete flush, there is some residue in the big resivoir, that i noticed, or perhaps the master cylinder needs replacing

shroom, my guess is that in the process of bleeding the system you flushed out a worn out master cylinder, and with the dirt gone, the seals are letting fluid by. No fluid loss means no leaks, the piston in the master cylinder is most likely just squishing through the brake fluid creating no pressure.