2003 Toyota Sienna Spongy Brakes

2003 Toyota Siennia Van

I replaced the front disks brakes & checked & adjusted the rear drums with a slight drag, without braking the lines, then after that, the pedal was spongy. It would stop ok, but keeping pressure on the pedal, it slowly goes to the floor. After some reserch, I replaced the master cyclinder (the reservoir is mounted on firewall, could not bench bleed because of this), bleed m/cyclinder on the the vehicle. I then bleed lines starting from furtherest wheel from M.C. With the same results. I repeated the above steps at least 10 times, still the the same spongy pedal. Its been 2 months & there has been no change for the worse or better. Any help would be great to solve this. Thanks