Break Pulsing Issue when I slam 'em

When I slam on the breaks hard I'm feeling a significant pulse through the pedal and it seems to be coming from the front breaks. No lights come on and it's dry pavement so I don't believe it's ABS. Also, the pulse is a little more aggressive and irregular for ABS. If my rotors were significantly warped I would feel some vibration under normal and aggressive breaking but I only feel it when I really lay on the breaks. It looks like there's plenty of meat left on the pads. I also don't see any leaks around the calipers. I don't know if this matters but it has been a few years since they've been bled and I know that should be done annually. Any Ideas?

How fast are you going when you apply the brakes? How far does the pedal depress?

have the rotors etc checked by a repair shop for true with a dial indicator and mike them for even thickness

Sneaky, As slow as 25mph or less and hit the breaks hard. extanker, No the rotors have not been checked. It just seems like something more dramatic than a wear issue but I have no skills--maybe it is.

Warped rotors is my bet. Mike

I think its the ABS kicking in. You don't need wet pavement for it to come on, just have to hit the brakes hard enough to lock them up and the ABS pump will take over.

This can be caused by buildup of brake pad material on the rotors, the first thing to try is re bed the pads, it is not usually the rotors warping that causes the pulsing but an uneven build up of pad material, take it out to an area where you can do this safely, take the car up to 60ish and brake hard but not enough to set off the abs slow to about 30, try this 4 or 5 times if they smoke a bit that is ok, just don't come to a full stop that will cause pad material transfer to the hot rotors, then drive around a bit until the rotors cool off, try to keep it rolling without coming to a stop while they are cooling down.

Should a light come on when the abs engages?

Quotes source post: 11216444

You will only get a light if there is ABS failure. On dry pavement my Volvo, Porsche and BMW R1200RT do the same thing during HARD braking. The Porsche is worse if I hit a bump. It will pulse quite a bit because the front is so light.

rwiii.......Has this always happened when you drive or is this the first time your hitting the brakes hard?

I used to track the car and just don't remember it happening on a 100MPH to 25MPH all-on braking deceleration. Maybe it is abs. I just ruled that out because of the dry pavement, slow speed and no light coming on.

It is the ABS. If there was a mechanical condition that was causing the brakes to pulse like you describe you'd feel this all the time, even under light braking. I do not know where you are but probably it is cool (or cold) where you are the tires are not as grippy as they would be in warmer temperature. The tires are probably not new either and have become harder and more slippery from the heat cycling. My the front tires on both my Boxster and Turbo get like hard plastic towards the end of their useful life and under moderately hard braking even on dry pavement the ABS can activate. Under normal -- street driving -- conditions brake fluid should be flushed/bled every 2 years. I have not noticed any issues with the brakes from letting the fluid go longer but the clutch in my Boxster was acting up until after I have the 2.5+ year old brake/clutch fluid flushed/bled.

The ABS light doesn't come on when the system is activated. I can't beleive you don't know this.

I was getting a lot of that at the track last week. Usually trail breaking while under lateral load. Seemed to go away when I pushed down harder on them. I just thought it was abs interaction???

Breaks = brakes Pulsing = ABS

Not to start a debate or anything, but on a (somewhat) technical forum like this, we should try to avoid talking about "warped' brake rotors. Truly warped brake rotors is such a rare phenomenon that I doubt anyone on this forum (even the racers) has ever experienced it. Raising it as a possible cause for anything doesn't really help anyone with their problem. Pad deposits on the other hand - we've all had that.

Really? Out of curiosity if not 'warped' what would you call rotors that have developed 0.020" run out that can be touched up true on a brake lathe?

Pad deposit is rare I think. I've seen and had plenty of rotors that had a pretty good runout that was easily measure with a dial indicator.

Nothing rare at all about pad deposits. I bet that every member of this forum has had pad deposits at some point. It's almost impossible (for me) to avoid pad deposits with something like Pagid RS19s for example. The fact is that the conditions required to truly deform a properly installed brake rotor are so extreme that I think only large jet airplanes and the Space Shuttle (RIP) encounter it with any frequency. I'll defer to the oft-cited Carroll Smith: "...in more than 40 years of professional racing, including the Shelby/Ford GT 40s – one of the most intense brake development programs in history - I have never seen a warped brake disc. I have seen lots of cracked discs, discs that had turned into shallow cones at operating temperature because they were mounted rigidly to their attachment bells or top hats, a few where the friction surface had collapsed in the area between straight radial interior vanes, and an untold number of discs with pad material unevenly deposited on the friction surfaces - sometimes visible and more often not. In fact every case of "warped brake disc" that I have investigated, whether on a racing car or a street car, has turned out to be friction pad material transferred unevenly to the surface of the disc. This uneven deposition results in thickness variation (TV) or run-out due to hot spotting that occurred at elevated temperatures." Edit: I realize that resorting to a third-party "expert" is a pretty lame form of discourse, but it seemed appropriate here.

Could it be related to the fluid? Today I was looking into what brake fluid to use for a flush, and found that viscosity can be very different between DOT 4 fluids. Cars with ABS should use a thinner fluid for quicker/smoother pulsing.

"....... This uneven deposition results in thickness variation (TV) or run-out due to hot spotting that occurred at elevated temperatures." What exactly is this supposed to mean?!?! Very blinker fluid kind of sentence.

I had a similar issue on my C4S with EBC pads. I attempted re-bedding them a few times and it didn't help. Finally, I took the took time to remove the pads and rotors one afternoon and gave both the pads and a good roughing up with 400 grit sandpaper. Problem solved, and it only took about 45 minutes.

Quotes source post: 11231774

Maybe I just have crappy luck, but I've warped a rotor on two cars over the years. One was a Ford Tempo, so I don't think it was due to the massive speed I had to bleed off. I do not think THIS thread is a case of warped rotor - still just sounds like ABS kicking in to me.

Quotes source post: 11216068

OP: One thing to remember is that when ABS is engaged, you are not stopping in the least possible distance. If you get to the point where ABS is engaged and then back off fractionally, so you are at the threshold of ABS (or nearly locked up on a non-ABS car), your braking distance is optimized. This is even more evident in wet/snowy/icy conditions where the distance you slide with every pulse of the ABS system is increased. Loading the brakes progressively (instead of "slamming" them on) is a better approach if you can manage it. This allows the front suspension to load, increasing the amount of grip for the front tires. If you slam on the brakes with no front-end load, there is less initial grip, so you will immediately begin to skid (or engage ABS). ABS is a great solution in panic stop situations - in a non-ABS car, the car would begin to skid, causing the braking to be much less effective and also resulting in total loss of directional control. ABS is a lifesaver, but like many driver aids, it isn't the optimal solution for all situations.

Thanks for that. I have only driven the car in inclement weather a few times. It is now my daily driver so I'm sure I'll get more of a taste for it. Yes when I felt it it was a panic stop and then I recreated it a few times. I don't make a practice of hitting them hard while tooling around.