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      09-19-2011, 08:59 PM   #54
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Drives: E30 M3; Expedition
Join Date: Apr 2007
Location: USA

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Originally Posted by wwjd15 View Post
I mean the braking power is not stable enough to give me confidence. For example, if braking at a long straight away, Brembo GT BBK and Stoptech BBK (even some cracpy stock ones) on my other cars are able to brake and stop at the same spot lap after lap. However, AP is not the same. Sometimes it stops earlier than usual, sometimes it stops later than usual, which is really annoying and force me to brake earlier than where it should be. It can be happened just about anytime during the day, doesn't matter cold or hot.
Originally Posted by mastek View Post
This is not right -- you either have defective parts on your BBK -- or your pads are defective -- or your fluid has air in it -- or your ABS pump is wrong -- or your brake booster is wrong.
Originally Posted by Martin_D View Post
It is the brake boost assist. This happens to fast drivers in the E9X M3. The system pulls vacuum from anywhere it can and you get an inconsistent pedal.
Since wwdj didn't mention anything about pedal feel, my money would be on ABS operation rather than the brake boost levels in this particular scenario where braking distances vary in the absence of difference in pedal feel. If brake boost was to drop off, you should have a solid pedal but can still brake with you stand on it. In other words, there should be a direct and very observable difference in pedal feel.

If ABS is not working properly in certain conditions (which could be an issue with the modified bias of an aftermarket system), you might not always feel something significantly different in pedal feel, but can experience significant differences in stopping power/distance. One can usually observe what I mean when trying to threshold brake at a bumpy braking zone vs. a smooth one on the same day at the same track. (Of course, braking on a bumpy section complicates traction as well, which also factors into ABS operation).

Anyway, I am speculating and there are many potential causes that might explain the inconsistency.