My 1988 190E 4cyl Brake light came on last week and rear brakes locked up. I replaced both rear calipers and brake pedal was really spongy. Over the weekend I replaced booster and master cylinder and both rear brake lines. There is plenty of pressure at master cylinder and front wheels and no pressure at rear calipers. I bled brake all around four times and no rear brakes. Anyone has any idea what I should do next? I had a mechanic help bleed the brakes and master cylinder and he's baffled too

Thanks for any advice