when you vacuum bleed, it is possible for air to get into the system through the bleed screw. don't ask me how this is possible, I just know that it is, but it never happened to me when i vacuum bled. A lot of people will tell you that pressure bleeding is preferable for that reason. I got a motive pressure bleeder a while back and haven't vacuum bled since. I would try to vacuum bleed the brakes again and see if it helps.

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2005 Mercedes C230k sport sedan, 6 speed
1987 Porsche 924S - 968-engined track car