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S197 Handling Section For everything suspension related, inlcuding brakes, tires, and wheels.

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Old 05-04-2013, 02:21 PM   #1
Darth Evader
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Question 06 GT - Brake Pedal Softens as RPM increases above idle

I have stock 06 GT with 44k miles.

I was wondering if anyone might know what would cause my brake pedal to become softer and travel further as RPM increases above idle. It only takes increasing idle RPM by 50-100 for this to become noticeable. For example, even the extra RPM from having AC is creating a mushy pedal.

Shouldn't the pedal become more firm since the engine's vacuum is decreasing?

I've bleed the brakes a few times now and it doesn't seem to have an effect and I'm not seeing any air bubbles in the bleed tube. The pedal is acceptably firm when in Park at idle.

Could the master cylinder be bad?
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Old 05-12-2013, 06:38 PM   #2
5.0Yeah!
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Have you tested the Master Cylinder, and do you get an ABS light when the pedal is depressed?
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Old 06-26-2013, 10:01 PM   #3
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I replaced the master cylinder this week, but the issue was unchanged. I also notice that pedal is mushier at stop lights in hot weather (above 80 F), regardless of whether the car is warmed up or not.

To answer your other question, the ABS does not turn on.

I wonder if the brake hoses are weak? I hesitate to use stainless because I don't race the car and I've read that stainless hoses shouldn't be used on a daily driver. I may just have to take another Mustang for a test drive to see if the brakes are the same...maybe it's just the nature of the beast.
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Old 07-02-2013, 10:23 AM   #4
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My guess is that your brake fluid is contaminated.

Did somebody happen to put DOT 5 brake fluid in the system? DOT 5 is silicon based and will do a lot of the things you describe in a system not designed for it. Never put DOT 5 brake fluid in your system, It is not compatible.

Either way, I would get the brake fluid flushed in the system. Air and water in the system can cause the same kind of symptoms. Since yours is an ABS car, it is not a DIY project. There is a special tool which cycles the ABS during the flush to get rid of the old fluid and air in the ABS. The nearby dealer charged me $95 to do a brake fluid flush a few weeks ago. Ideally, the brake fluid should be flushed once per year - although street cars usually go a lot longer. If the original fluid is still in your 2006, it is way overdue.

Last edited by JThor; 07-02-2013 at 10:29 AM.
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Old 07-03-2013, 10:27 AM   #5
Norm Peterson
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Quote:
Originally Posted by Darth Evader View Post
I was wondering if anyone might know what would cause my brake pedal to become softer and travel further as RPM increases above idle. It only takes increasing idle RPM by 50-100 for this to become noticeable. For example, even the extra RPM from having AC is creating a mushy pedal.

Shouldn't the pedal become more firm since the engine's vacuum is decreasing?
I suspect that you have a small vacuum leak somewhere in the power assist line (probably closer to the PB booster) or possibly in the booster itself or its diaphragm.

You can check this yourself if you have access to a vacuum pump and a gauge (you'd be watching for the vacuum level on the gauge to steadily drop after briskly pumping it up).

I think if you pumped the brakes a few times on any car with vacuum assisted brakes without starting the engine (to ensure that no PB vacuum was present), then held your foot on the brake pedal as you started the engine that you would feel pretty much the same softening and additional pedal travel.


Ditto on flushing the fluid. A conventional bleed with the pedal or a simple gravity bleed will clear everything in the system except for parts of the fluid circuitry within the ABS unit. For fluid, look for higher wet boiling points first, then high dry boiling points. I'd preferably use a quality DOT 4 rated fluid, or maybe Ford's own OE fluid. Stay away from any minimum-spec DOT 3 fluids that you might find for cheap at the parts stores. FWIW, I'm running ATE Amber, sourced through Sam Strano . . . . (over a long enough time the blue stuff can stain plastic making the fluid level in the reservoir harder to see, not to mention that it is technically an illegal fluid for use in Florida).


Norm
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Last edited by Norm Peterson; 07-03-2013 at 10:43 AM.
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Old 07-03-2013, 10:27 AM
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