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Bad Master Cylinder or Air in Lines??

Hey folks, I'm new to the boards, but am hoping for help from the collective wisdom on these boards...

The short version: while my wife was away for the weekend, I decided to do her a "favor" by bleeding the brakes on her car. While doing so, however, I believe I may have allowed the reservoir to run dry, thus drawing air into the system - the brake pedal was very firm when the engine was off, but when the engine was running the pedal was very soft and would drop to the floor.

So I removed the master cylinder from the car, bench bled it, and reinstalled. Then with the help of a friend, re-bled all 4 wheels. There was no improvement. So we made 2 more passes at each wheel to bleed it, and have gotten to the point where no air is coming out of any calipers.

And yet the pedal is still the same - firm when engine is off, and drops to the floor when engine is running.

So how do I know if I have a bad master cylinder, or if I'm just failing to get the air out of the system? I know that sometimes the pumping action of the cylinder during the bleeding process, combined with grit and schmootz (technical term) in the old brake fluid can gouge the bore and cut the seals, thus resulting in a bad master. But how do you know? I'd rather not spend $100+ and find out that it was just air in the system...

Thanks for your help!! I need to get this sorted out today before my wife gets back tonight!


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