problem bleeding brakes

I had a replace a leaking mater cylinder. I bench bled the master cylinder, reinstalled it and began trying to bleed the brakes. I opened the rr bleeder screw. I had someone try and pump up the brakes. After about 30 tries I never got the first sign of any brake fluid. I then tried the LR and couldn't get any fluid to come out. I then tried the fronts. They bled easily. What am I doing wrong? I little more info, 1972 Dart, 10" drums all around and manual brakes. Thansk for any help I need it.

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