No brake pressure on front circuit after master cylinder replacement

[LightlyToasted]

2007 BMW X5 E70 4.8i.


Original problem was a hard brake pedal despite good vacuum at booster so I replaced the booster, which involves removing and reinstalling the master cylinder then bleeding the system (including DSC bleed via ISTA). Brakes had boost again, but pedal would slowly sink to the floor. I suspected that when I bench-bled the master cylinder it may have stressed the old seals, so I admonished myself for not replacing the master cylinder the first time and went ahead and installed a new master cylinder.

After thoroughly pressure bleeding, DSC bleeding via ISTA, and 2-person manual bleeding the pedal will sink almost to the floor with minimal resistance, as if there were a problem with the (just replaced) master cylinder. There are no bubbles in the system and no brake fluid leaks anywhere. When I pressure bleed I get clear fluid at all 4 corners so the circuits aren't blocked. When I 2-person manual bleed, no fluid comes out of the front brakes.

Since the front brake circuit is not blocked (pressure bleed works fine), the problem must be at the master cylinder which is not supplying pressure to the front brake circuit. Any ideas? Could there be air in the front circuit portion of the master cylinder? Bad master cylinder? To be clear, I bench bled the cylinder before installing it (even though the BMW TIS doesn't require it), and additionally I slowly cycled the brakes while pressure and DSC bleeding as per the TIS.