Hey guys i am usually pretty good at diagnoses, First i have a 99 chevy tahoe and the rear brakes went out (drum brakes) and the brake pedal traveled to the floor. So i put new shoes and both wheel cylinders in bleed the brakes and still same thing pedal still went to the floor but after i let the pedal back up the brakes would seem fine untill next time i stopped same thing to the floor. So then i replaced the master cylinder, i bench bleed it and installed it and then bleed the system. Now the pedal goes down 3/4 of the way and is still not really any better than before. When i hit the brake there is not much pressure but once again when it comes back up and i push it down again it feels normal. I dont know what else to try maybe the power booster? i don't know Please help me out.