Brakes Locked on Freeway

I took out my 1999 996 recently on the freeway and about 20 miles into the trip all 4 brakes engaged forcing me to limp off the freeway. No warning lights were visible on the dash (abs/psm etc). Took it to a well respected indie mechanic and he resurfaced the rotors and did a complete brake job with new fluid however the following day the brakes locked again after a short 15 mile trip. Anyone have an idea on what could be causing this behavior? Note that I had the brake booster replaced about 18 months ago because the brake pedal was not returning to it's original position. Indie didn't find anything suspect with the new brake booster. I have the following aftermarket parts installed: Dectane led rear brake lights Wheel spacers Fister exhaust Any help would be greatly appreciated as it's been at the mechanics for over a month now and he is still trying to diagnose the problem.

By locked, you mean they're getting applied at some level and not releasing? "Locked" initially made me think they were locked up, but obviously you wouldn't have been able to limp off the freeway or get to your indie. Pull your ABS/PSM fuse. It sounds to me like something with your PSM has gone haywire and is applying the brakes inappropriately. I'd prefer driving with no ABS over the possibility that the car will unexpectedly slam on the brakes at an inopportune time... I'm not in the habit of second-guessing someone else's mechanic, but I'm having a hard time understanding why new brakes and rotors would have seemed like an appropriate solution for "my brakes are coming on by themselves." Check the cap on your brake fluid reservoir - I saw a post that indicated that if the reservoir gets pressurized, the brakes could potentially apply themselves. However, you said your indie flushed the system, so it seems less likely that there is water flashing to steam and pressurizing the circuit. I'd take a long, hard look at the PSM/ABS system.

Perhaps I should of opted for a different term other than locked to describe the behavior. They didn't completely lock up however they engaged to such a degree that I had to really rev the engine hard just to get it off the freeway safely. The mechanic did a complete brake job and bled the lines, added new fluid, and resurfaced the rotors. He may have initially suspected that their was air or water in the lines (much cheaper than replacing an abs control unit). He mentioned he's looking at the abs system now as a possible suspect however 30 days in and just an unconfirmed diagnosis has me wanting to pick up the car and take it to another mechanic.

Does the problem reproduce for the mechanic? I'm guessing no, and it's just been sitting on his lot for 30 days because he can't come up with anything - but I don't want to make assumptions. I don't envy the position you're in - trying to troubleshoot a condition that makes the car potentially unsafe to drive...

Yes, the mechanic was able to reproduce the problem.

So, what did he learn? Was the ABS pump running, did he monitor the pressure in the brake lines and/or the reservoir, was the brake pedal still low (as it was when the booster was replaced), anything at all?

Heading over there now as I have been unable to reach them via phone and they won't return my calls. Not sure what's going on over there but as soon as I know I will post here and hopefully other members will find the information of value. Thanks again for the help.

Sounds like a great mechanic you found! When you finally get the car back, expect this: Finally, to improve value of this thread:

Where are you located? This sounds like something a bunch of PCA guys could be enticed to investigate with the promise of cold beer and hot pizza...

If it's sporadic and affecting more than one wheel, probably isn't mechanical. I had a brake caliper lock up on my BMW 2002 on the freeway and that never let go.

nice air bags

I vote for a master cylinder issue.

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Same here. In my mind the only thing that applies pressure to the brake hydraulic system is the master cylinder. The piston is probably getting stuck in the cylinder and not releasing properly. Brake booster is only there to provide assist with brake pedal effort.

"Note that I had the brake booster replaced about 18 months ago because the brake pedal was not returning to it's original position. Indie didn't find anything suspect with the new brake booster." I'm gong to say that it's something there. My friend had an old Jaguar E Type, replaced the brake master cylinder or booster (if it has one), and had very similar issues that you describe. In the end, the part was not correct, and when everything heated up after driving the car, due to thermal expansion of the components, clearances became interferences, and brakes were now applied without touching the brakes, just like your experience. He ended up shortening the MC push rod which kept clearances as clearances, so there was no brake pressure applied. I'm not saying that's your exact problem, but I would not be surprised if it was something like this.

Is your 996 a C4? The master cylinder/calipers are in theory passive components, they should not have the ability to generate braking force without a separate input, mechanically. The brake booster could in theory pass the vacuum force onto the master cylinder. Typically the diaphragm in it, or the seal between it and the master cylinder will break. These can cause some “pull” on the master cylinder to apply brake force and I think your problem may lie here. You could disconnect and cap the vacuum line to the booster to test it. In addition, pulling the ABS fuse would also help eliminate that as the cause of the problem. I would start with that. These just happen to be easy to check, without the need for much labor or additional parts. Good luck and keep us posted!

Had a similar event years ago (not in a Porsche)......the root cause was bad brake fluid (moisture) it took a thorough break fluid to solve the issue.... Ahmet, nice to hear from you !!.........

Hey man how you doin! It’s been a while, still got the 996 cab in Raleigh?

dude....go over to the shop with a tow truck and get it out of there.....try a dealer

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Yes, and enjoying it like the first day ! I do not want to hijack the thread.....the PO needs to get his car out of that place !!

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+996. or maybe another indy if one is nearby.

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If one caliper is sticky and is getting hot enough to boil fluid, I suppose maybe in theory it could generate enough pressure to mess with the entire system, but I've never heard of boiling fluid having this result. Since OP's first trip to the garage included a fluid flush, this seems even less likely... The one good thing you've got going for you is that it can be reproduced - that's a whole lot better than an intermittent problem that cannot be recreated when someone's actually trying to diagnose the issue. I know this is Capt. Obvious speaking, but...don't put any more highway miles on that car until you know why it's braking unexpectedly...

I say the master, it's holding residual pressure

All good notes so far. Brake Booster would be my first suspect. Had the same symptoms on my Boxster. Is the car stored outside by any chance? If the drain plugs are clogged, that compartment can be flooded really easily and water can enter the booster, ruining even one that is 18 months old. After that, the mechanical inputs need to be run through one-by-one. The pedal assembly can also bind at the firewall, although that is pretty rare. Early Boxsters had a different brake booster push-rod that needed to be updated along with the Brake Booster that could cause binding in the pedal assembly. Not sure about the early 996. As others have said, go get the car.

I would say if it was just one wheel locking up it would be probably be the rubber brake line, but all 4 wheels the master or the abs. -Jason

I would go back to the original problem of brake pedal not returning all of the way, water in the booster? A sticking pedal would cover the fluid intake at the master cylinder. Then anything that would heat the fluid could cause some pressure in the system, because it can not expand back into the reservoir.

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In do a lot of DIY on my car but if this was happening on mine I would have to take it into a Porsche Garage for them to sort out my problem. I advice you do the same. To hell with your so called indy. Get someone who knows their job to do your work. My car is more valuable to me than to be letting a so called INDY eff about with it at my expense. Take it into the guys who know their job.

WTF Hurdi, what makes you think the trained monkeys at the marquee are any better or smarter than an Indy trained monkey? In fact many of the indy's are/were factory trained and some of them have master status

Keep in mind that he's in PA - in some parts of PA, the folks you find working in garages are a few chromosomes shy of the genome. cooter.jpg

Having operated a shop where we�ve fixed cars not properly diagnosed by dealers more than once, I have to agree. In general you may have better luck at a dealer (potentially), but IMO the only thing you can reliably count on would be a nicer wait room. RE: Brakes. Stuck caliper/boiling fluid COULD happen, but I imagine that�s a lot less likely than a failing booster. I haven�t read anything on this thread so far that IMO could reasonably result in an intermittent application of all 4 wheels aside from an ABS/PSM fault or a bad booster. Side note: you can push fluid from the calipers up through the master. I suspect a reasonable amount of excess line pressure would likely vent back into the reservoir and not result in significant and sudden application of the brakes. Educated guess about what�s going on: This is a 99 model car with a regular cable throttle (otherwise he can�t be on brakes/gas for more than 3 seconds), therefore the car does not have PSM. No PSM reduces chances of this being an ABS problem. (The ABS system is much simpler than PSM). Anyway, car has a faulty booster, which when the diaphragm is aligned properly, uses the vacuum supplied to pull the master cylinder actuation rod inward, applying brake pressure.

My advice is take it to a PORSCHE garage not an indy. I do all my own DIY but things I cannot fathom or a job more complicated than I can do I take mine into a PORSCHE garage. Something is seriously wrong so let them fathom it out for you. Money well spent in your case.

I would jack the car up and spin all four wheels, your problem should show itself real fast.

Hurdi is on fire this week.

Question: Where do all great Porsche mechanics go after working at a Porsche dealer for years and years ??? Answer: They go to work for an independant garage or start their own The only service I would take my car to a Porsche dealer for is programming keys and such that can not be done anywhere else due to proprietary security codes that can't be gotten anywhere else

I _think_ Hurdi believes you should take it to the dealership, but I'm still unclear.

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He says to take it to a "Porsche Garage" and a "PORSCHE garage." What's confusing about that? Clear as a brick.

Like this kind of dealer?

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I don't get it

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Yep just like that one. Maybe the INDY who used to work there was 'let go' for workmanship not up to PORSCHE standards. Then he had the great idea of starting up on his own saying that he at one time worked for PORSCHE. His status would sky rocket to the point that lowly 996 owners would flock to his place in an attempt to have work completed on their car at a cheaper price. He keeps the car for a month, still trying to fathom out the problem which in turn starts this thread up. Then all the 'would be mechanics on this board' try to solve his problem with this, that and the other suggestions. My suggestion seems to be the best one so far in solving the posters dilemma.

Sure go to the dealer for your $300 oil changes and the pretty barista Most of the Indy's I know have forgotten more than many of the dealer techs know, at least on older cars. Several of them achieved Master status from Porsche. Are you saying they don't know what they are doing? Maybe the tech at the Porsche dealer is very new and learning about the cars

Just wanted to say thank you to everyone that responded and provide an update. Looks like the brake booster was installed incorrectly by a previous mechanic and the pin was not correctly aligned. I didn't notice this for quite some time because the car is not a daily driver and has not been taken on any extended trips until recently. The 30+ mile extended trip was sufficient to cause the brakes to engage. The mechanic replaced a few o-rings,adjusted the brake booster, and took several 30+ mile trips to see if he could reproduce the issue. Just got home and looks like we are good for now. Again, thank you to everyone that responded.

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So was it the INDY mechanic who had your car for a month that solved the problem or did you take it into someone else?

Hey Whynot, great to hear that the problem was found and fixed! Thanks for the follow-up post.

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Have KC give it a go. He's still polishing his stand-up routine, but is at the 30-grit range of sandpaper still.

Ditto, thank you for sharing the outcome. It’s good to know how a puzzle works out, and glad your car was fixed.

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OMG, a mechanic who doesn't work at a Porsche dealership fixed the car? What a freaking miracle. Everyone knows that the only capable Porsche mechanics work at dealerships. OP, congrats on having a reliable car once again.