Vibrations under braking

Hello, I have an ongoing issue with vibrations under braking on my 2001 996.1 C2 currently at 230.000km and after 3 sets of rotors and pads in the past 2 years I have not been able to completely resolve the issue... So this will be a long one, please bear with me: 1st replacement: The used car I bought was standing still for some time and developed the well known rusty inside of the brake rotors (discs) which were heavily vibrating under braking. The front rotors and pads were replaced with ATE and seemed to work fine for about 5.000km. After that the shaking returned although not nearly as bad as it was in the beginning. It gets worse when the brakes get hotter and it's most noticeable when braking inside the corner when the wheel is not straight. No track days, only quite conservative spirited driving on public roads. 2nd replacement: The indy shop convinced me the guys who previously installed the new rotors and pads did not clean the surfaces of the wheel hubs and when they mounted the rotors they were of course not completely straight and this caused them to become slightly bent. They recommended the Brembo brand and replaced front rotors and pads with Brembo. Again it all seemed to be fine for about 5.000km until the issue was back. When brakes got hotter the problem was worse. I went back to the shop and the owner said that the brakes were not fully bedded in and I went with him on a ride where he probably burned them a bit but anyway they were extremely vibrating/shaking, basically not drivable after he was done. When they cooled down they were drivable but the problem remained. 3rd replacement: I decided to go to a different indy shop where they measured the rotors and hubs and found a deviation of about 0,05mm on one of the hubs which lead to (I think) about 0,2mm deviation on the rotor. All four hubs were removed, bearings replaced and the hubs resurfaced. Zimmermann rotors and pads were installed on all our four corners. There was absolutely no issue with the vibrations when braking in corners and everything was looking good. As if on schedule after 5.000km the vibrations have come back. They are not super bad but they are annoying and noticeable when braking in corners. I have not yet made them super hot to determine how much worse they get. I did however perform a few short stops as a re-bedding procedure but this did not give the desired result. I will highly appreciate anyone who has had the same kind of issue and will share their path to a solution.

Sounds like you need a calipers rebuilt

Unfortunately they are new. I forgot to mention that all four calipers and brake lines have been changed after the first exchange.

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Sorry about your dilema Out of curiosity, two quick questions. (1) Do you have psm? (2) Sound like its happening only under braking during turn in, right? What happens under very heavy braking from a high speed in a straight line?

I have psm. The vibrating is more noticeable in the corners but it can be felt in a straight line at higher speeds too.

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Just to be clear, there's zero vibration until you apply the brakes?

Absolutely no vibration when the brakes are off.

I'll first say I don't know whats going on. I would look at tires and alignment, and steering box and tie rods. See if you can detect any play anywhere. It's something, and maybe not the brakes. Have the control arms been replaced?

Can you determine what exactly is vibrating..? Does the pedal pulsate..? does the steering wheel shake..? Do you just feel it in the seat..? So, you spent $3000 on new calipers all around..? Sure sounds like a sticky piston causing eventual rotor warpage. Has anyone checked the runout of your rotors...? Doubt its got to do with the psm/abs, but just to rule them out,,,, Maybe try removing the abs fuse and try straight line braking to see if your vibe is still there...?

Hubs bend and stay bent when wheel bolts are overtightened and/or not tightened to the same torque. This will cause the disc to have a slight runout which when new doesn't cause the vibrations as the runout presses the pad on one side of the disk into the caliper while at the same time the pad on the other side of the disk is pushed out from the caliper and 'follows' the disks runout. As such You don't feel pulsing in the brake pedal, no varying deceleration nor any vibrations. However, over time the disk wears unevenly around its entire circumference. Looking through the wheel spokes onto the disk imagine the face of a clock. Now the wear I'm describing has the disk worn thinner at 12 3, 6 and 9 o clock whilst at hours in between it hasn't. (It could be 12 and 6 or just one thickest and one thinnest spot around the disk but you get the idea). This is when you start noticing vibrations. Replace the front hubs, resurfacing is no lasting repair. I recommend replacing disks and pads at the same time and stick to the brand that was on there as to keep that a constant factor in the process. If it helped replace the rear hubs, disks and pads too. No need to replace wheel bearings if the hubs can be replaced without damaging the bearings . Of course before throwing parts at it it is wise to measure disk thickness around the disk as well as run out. Keep in mind though that the wheel is not mounted and as such your not measuring what is an assembly when driving. Come to think of it: Check flatness of the hub faces of your wheels too. If they are off, the hubs will also get bent when the bolts are tightened even when evenly and to spec. These are not some random thoughts of what it could be that you might or might not believe but based on analysis of the phenomenon you're experiencing. Ignore any replies that suggest PSM intervention, tyres, brand of pad and disks or mismatch of those, bedding-in procedures having been wrong or re-bedding being required as it's NOT the root cause of your troubles. Neither is worn suspension or steering components as those do NOT play-up only when braking and certainly not only after 5k of having replaced disks and pads. This sounds I'm full of myself which in this case I am... Edited to reflect thprf's info.

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Hubs cannot be replaced without damaging bearings. Removing the hub from the suspension carrier destroys the wheel bearing.

I thought about the steering components but in fact there are no issues until I get to about 5000km and even then the problem of a vibrating steering wheel is only under braking. Yup, replaced all four calipers and brake lines with original porsche. I will try to shut off psm and abs but I doubt it's the source. Doesn't cost me anything to try it though. The idea now is to change the front pads with very aggressive ones and use them for a few hundred km to try and smooth out the rotor surface. As I mentioned before the vibrations are not extreme and they are not felt on the brake pedal, only on the steering wheel. It's just annoying because it gets worse when driving down a hill when the brakes will get hotter and the vibrations will increase.

Perhaps the master cylinder is sticking and forcing a brake to drag. I would use an infrared temperature reader to check the temperature of the calipers and rotors when the vibration is at it's worst. Pull over to the side and check all 4 wheels.

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I had so many "red herrings" in my life. The 5000km may be one of those. Going downhill changes the loads/angles the front end. Maybe that makes it worse

Too be clear there are no symptoms until 5,000km and going downhill makes it worse. How hard are you driving the car? Track, spirited side roads, etc? First I would get the Indy or someone else drive the car to feel the symptoms. Based on what you have described something is abnormally wearing your front rotors and pads. A warped rotor would be felt in the brake pedal and the steering wheel. As already mentioned by "theprf" borrow or buy a cheap infrared pyrometer to measure rotor temps. If the front pads are wearing abnormally you should be able to see the difference. My inners normally wear more than the outers, but not significantly. I was not aware that a master cylinder (or ABS) could cause one caliper to drag. While track pads have a higher coefficient of friction than street pads, they won't smooth out a rotor. Another somewhat random thought. Your driving style and/or pads may be leaving uneven deposits on the rotors and takes time to build up. Try an aggressive "bedding" of the pads which requires multiple stops at speed to remove and then deposit a new transfer film on the rotors. From my experience typical street driving and OEM pads do not create a transfer film because the temps are never high enough. If you invest in a set of "track" or multiple purpose pads like Ferodo DS2500 definitely do a "bedding". A real mystery, keep use posted. I fear you may have invested in new parts that you may not have needed.

I think the master cylinder is fine, if it were applying more brake power to the left or right the car would "steer" during braking. It stays straight. I've been measuring temps on the brake rotors and there are no significant deviations between left and right. Fronts are always about 50-80�C hotter than the rears but I think that's normal. I only do spirited driving and the maximum I measured my rotors after a very hard session was 250�C. The indy shop says that they've been having many issues with OEM rotors and are lately installing only original Porsche ones. I am leaning in this direction as well once I make sure that my hubs are absolutely straight. I will make an update once I get to the bottom of this but in the between time any suggestions are welcome.

I’m just spit balling but did you check to see if the rotors are reversed

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I had a wobble issue once that I'm convinced was due to the rotors metallurgy. They were true but caused a wobble. New rotors solved the problem. I assume the coef of friction was not uniform

Sure, it could be alot of different things. With everything you have done on the issue and said, if it was my car (which I know it isnt) here is what I would consider doing. First thing is make sure that your car insurance is paid up and it's insured for the replacement value you want just in case. Then I would find a big open parking lot or straight road that had plenty of room for run off in case my brakes failed. Not to mention with no one around for safety. Yep, wear your seatbelt and tighten it up as well by locking it in. Then I would run a number of high speed rip your face off braking sessions that would heat up the brakes and scrap off any crap you may have on the rotors. Make sure your tires are warm so you have some grip or you will be in abs heaven. If that doesn't cure it, then take it in to an indy that works on Porsches who can drive the car and go thru the checklist. Sure, your rotors could be warped, but I doubt it. What can be pretty common is that after very spirited braking on the track and street, folks may then sit in their cars at a standstill with their foot on the brake. This is not good because it can transfer stuff from the pad to the rotor and cause some vibration. Same can be said for using the emergency brake. This is why we tell folks when they get off the track to drive around the paddock slowly and cool down their brakes before they use them to park. Hope it helps. Keep us posted. Quick note: don't do this unless you know what you are doing and know your brakes. It's one of the first trainings (autocross) we put people thru before they get on the track. Why, because you never really have to use the brakes hard on the street and therefore, very few folks have ever experienced what rip your face of braking actually feels like. It can be unnerving. So make sure you are doing it in a safe location and the pavement is dry and safe. Second note: every time I had vibrations during braking, it was due to crap getting transferred from the pad to the rotor. I felt the vibrations the most going downhill. After a few heavy braking sessions, it went away. Because we don't know your driving habits it is tough to know if this may be the cause. But it doesn't cost you anything other than time to go thru the exercise to check the box.

Lots of very good advice here to help solve the vibration that re-occures after 5k especially the post from hardtrailer that is detailed technically and is spot on so consider it heavily on every aspect.. The only thing I can add to the other great comments is that some of the hardest technical problems can have very simple beginnings that are easily overlooked.For Example :: Never put the car away wet ... At the Porsche Dealership I have seen countless cases of brake vibration after a period of time from customers washing the car and putting it away wet... The cast iron rotors will surface rust overnight all the way around except in the 7-8 inches where the pads/calipers are sitting, This rust will be quickly removed as soon as the brakes are applied and usually causes no harm, but if these process is done repeatedly the constant spot rusting and removal will effect the thickness of the rotors in spots and cause hot spotting and vibration... I have seen cases of this at the dealership with ( new or "like new" cars) surface rust so bad the car will not physically move from where it is parked, or will slide one wheel/tire when moved until it can break the rust loose...Those are worst cases but light cases of this same thing can happen without notice and repeated events will lead to vibration after a period of time/number of events..... Just something to keep in mind....these cast iron rotors will surface rust so fast and ANY moisture even close to them will rust them...

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Yup, have experienced the exact same thing on many Porsche cars I've owned. Happens fast too. I can leave car out in driveway after wash for a few hours, and when trying to pull away or go back in garage,,, you can feel/hear the brakes locked up then break free....