@bashby: You were right, it was air in the system. New air was getting introduced through the slave. I was trying to pull it, finally got pissed with the insane hydraulic line routing, and ended up cutting it out.

On a venting note I'll say: It would be REALLY F**KING EASY for these systems to be designed so that the air travels to the master cylinder, and put a release valve on it. It's completely insane to think this could be overlooked by any engineer worth his salt... But hey, with 100$ shop fees, maybe it wasn't overlooked after all...