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Uneven brake dust

Old 10-11-2018, 07:57 PM
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Wharf Rat
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Default Uneven brake dust

Why I am getting a ton of brake dust on the rears (especially the drivers side) and almost none on the front. Pads are brand new Hawk Ceramics. Brakes seem to be working fine. I bedded them in properly and the pads now have about 150 or so miles on them. Any ideas???
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Old 10-11-2018, 08:18 PM
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Tom400CFI
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You mostly brake lightly? During light braking, I believe that the bias is more toward the rear so light braking COULD cause more rear wear and dust.

I may be wrong; the bias may be 1:1 initially, the drop in the rear as pedal pressure increases. I swear I've read that rear applies first and more initially, for vehicle stability.
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Old 10-11-2018, 09:13 PM
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Your real calipers are leaking and you have a mix of brake dust and brake fluid on the wheels.
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Old 10-11-2018, 09:31 PM
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Other than bedding them in it's been driven fairly tamely since the pads went on. If the rears really engage first I guess that could be it. Although I thought these cars in general had too much front brake bias (thus the need for the DRM brake bias spring to get MORE rear brake).

No fluid or leaks back there. It's for sure just dust.
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Old 10-12-2018, 12:25 AM
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Originally Posted by Wharf Rat View Post
I thought these cars in general had too much front brake bias (thus the need for the DRM brake bias spring to get MORE rear brake).
Brake bias changes with pedal pressure, so at high pressure (max braking), there is too much front bias (especially with upgraded front brakes. But bias at low line pressure (light braking) is different.
Here is an example (not a C4), from Stoptech's site:



And here is the accompanying text:

Looking at the diagrams, one can see that it is possible to design both a Type B system and a Type C system that ultimately give the same brake balance at the point of maximum deceleration. (Note that Figures B and C both generate 950 psi of rear brake-line pressure when the front brake-line pressure is at 2000 psi.)

However, one can also see that Type C systems—those that use proportioning valves—can bring us closer to optimum balance at lower deceleration levels. This benefit is relatively meaningless in a racing application, as the vehicle is always operating at maximum decelerations, but it is of great advantage on the street.

In so many words, the proportioning valve allows us to drive around town under optimized brake-balance conditions (good for front brake-pad life) but also keeps everything in check when we need maximum braking (good for stability).

Due to their compact size and relatively low cost, these devices can be found on nearly every vehicle which requires rear brake pressure reduction to achieve optimum brake bias. Typical passenger cars and production-based race cars fall neatly into this category.


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Old 10-12-2018, 11:19 PM
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On my 96 GS I have more brake dust on the front rims then the rear.
However I wore out the rear brake pads at 38,000 miles.
The front still has the original pads at 45,000 miles, and still has more material left on them.
The rotors are all original too.
I do downshift a lot and I would say I am not too hard on brakes. Yet overall I am an aggressive driver.
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Old 10-14-2018, 09:43 AM
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….. The pressure differential valve may have been tripped to prevent fluid loss from the front brakes … this happens often during bleeding … and you are braking with the rears only … you might try a panic stop in an open area to confirm that all 4 brakes are actually working …..
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Old 10-18-2018, 01:49 PM
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Originally Posted by C409 View Post
….. The pressure differential valve may have been tripped to prevent fluid loss from the front brakes … this happens often during bleeding … and you are braking with the rears only … you might try a panic stop in an open area to confirm that all 4 brakes are actually working …..
Curious, if this happens how do you reset?
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Old 10-18-2018, 03:34 PM
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It resets automatically after you replenish the fluid, bleed and then step on the brake pedal.
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Old 10-19-2018, 01:56 AM
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The fluid was done by the PO last year at a GM dealer so that should be good to go. I'm going to play around with my temp gun this weekend and see what the operating temps are.
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Old 10-20-2018, 02:40 PM
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The brakes all checked out. Drove around for about 10 miles at without using the brakes at all at 40 MPH and all 4 rotors were about 20 degrees above the ambient temp. Next I did 10 normal stops from 30 mph. Front rotors were running around 230 degrees and rears around 170. Then I did 3 panic/lock 'em up style stops from 75 and the fronts were around 400 and the rears were around 300. That all seems reasonable to me.

Only thing I can think of is the rotors have about 35k miles on them (I think anyway). I did not have them turned. Is it possible that the front pads are fully bedded in and the rears aren't yet (like still wearing into the exact shape of the rotors)? Still less than 200 miles on the new pads. I never noticed this on the old pads but I also just had my wheels refinished so I'm sure I'm looking at them more. Other than that I'm at a loss.
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