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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!


  • edited September 2009
    Did you also bleed out the ABS module? If you let the brake reservior run dry, you probably forced air into the ABS module. Do you have a manual that has instructions to bleed the module? If the module is also clear, you may just need a new master cylinder.
  • edited September 2009
    While running pump the pedal until it is firm. If it holds in the same place it is air in the lines, if it slowly descends to the floor it is the master cylinder.
  • edited September 2009
    No, I didn't bleed the ABS module... I have the Factory Service Manual for the vehicle, but it makes no mention of any bleeding procedures for the ABS.
  • edited September 2009
    When running, the pedal doesn't "pump up." I've pumped it as many as 20 times, and it never really gets any firmer.
  • edited September 2009
    You have air trapped in the lines.

    Try bleeding the brakes this way... With somone helping you open the bleeder farthest from the master. Have the person in the car push the brake pedal down BUT not all the way to the floor, have him hold the pedal in that postition while you close the bleeder, after it's closed he can release the pedal.

    Repeat this 4-5 times and then move to the next bleeder.

    You will notice big burps of air coming out of the bleeders but you might not get the air burps until the 4th or 5th time you open the bleeder.

    After you finish all four try starting the car to see how the pedal feels. If it is better but not quite right you might have to bleed the brakes again.
  • edited September 2009
    Yeah, that's exactly the procedure we followed. We did roughly 4 "burps" at each wheel until no more air came out, and then moved to the next wheel. And once we hit all 4 wheels, we started the car and tested the pedal feel. And then we made 2 more entire revolutions around the car using the same procedure...
  • edited September 2009
    Monday bump... Any other ideas out there?? :)
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