mastercylinder question?

[lahnen]

its been a long time since i have had to mess with mastercylinders, but i'm on my second new one on my suburban. after bench bleeding and installing, i'm having problems getting the rears to bleed out and to work. when pushing the pedal, fluid is shooting back up into the front brake reservoir. i didnt think this was supposed to happen, but i got 2 in a row from 2 different stores now that do the same thing. should i try a third mc or look into something else. everything is brand new except the booster and porp. valve, but they appear to be workin properly.