just had this same problem; turned out my new rear calipers were in the wrong boxes, and I put the left on the right and vice versa, with the bleeders at the bottom. switched them to the proper sides, with the bleeder at the top, and no more troubles.

Also, if you let the reservoir run dry while you had the calipers off, you may need to also bleed the MC, OR, if your MC is really old and crusty and you pumped too far down while bleeding, you might have screwed up the seals inside.

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1976 Mercedes 240D, unknown mileage
1992 Dodge/Cummins 4WD, 284k