I've tried bleeding the system when I replaced the brakes and rotors. I have found that not to help. I have looked at the schematics and do not understand what type of sensor is causing the VDO to display. I have seen suction hoses pull air into a hydraulic system without an external leak and I am beginning to think this may be the issue, but as stated previously, how is the ECM being told there is an issue? Pressure, Suction, or a float switch?