Brakes Locked on Freeway
Thread Starter
Rennlist Member
Joined: May 2009
Posts: 8
Likes: 0
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.
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.
Race Director
Joined: Sep 2012
Posts: 10,424
Likes: 229
From: SOcialist republic of CALifornia
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.
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.
Thread Starter
Rennlist Member
Joined: May 2009
Posts: 8
Likes: 0
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.
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.
Race Director
Joined: Sep 2012
Posts: 10,424
Likes: 229
From: SOcialist republic of CALifornia
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...
I don't envy the position you're in - trying to troubleshoot a condition that makes the car potentially unsafe to drive...
Race Director
Joined: Sep 2012
Posts: 10,424
Likes: 229
From: SOcialist republic of CALifornia
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?
Thread Starter
Rennlist Member
Joined: May 2009
Posts: 8
Likes: 0
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.
Thanks again for the help.
Trending Topics
Rennlist Stories
The Best Porsche Posts for Porsche Enthusiasts
I've Written 500 Rennlist Articles: Here's How Porsche Has Changed Along the Way
Joe Kucinski
10 Most Unnecessary Porsches Ever Built (And Why We Love Them)
Verdad Gallardo
Porsche 911 GT3 S/C vs 718 Spyder RS: 10 Categories, One Winner
Joe Kucinski
This Builder Is Turning Heads With Its Slantnose 911 Creation
Verdad Gallardo
Porsche 911 GT3 Artisan Edition Pays Homage to Japanese Culture
Verdad Gallardo
Porsche Reveals Coupe Variant of the Electric Cayenne With a Fresh Look
Verdad Gallardo
10 Porsche Colors That Have More Personality Than Most People
Verdad Gallardo
Guntherwerks' Final Speedster Creation Is the Ultimate Porsche Restomod
Verdad Gallardo
10 Reasons I Hate Going to the Porsche Dealership (& the 1 Reason I Stay)
Joe Kucinski
RL Community Team
Rennlist Member
Rennlist Member
Joined: Apr 2010
Posts: 6,282
Likes: 1,506
From: Ormond Beach, FL
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.
Rennlist Member
Joined: Jul 2013
Posts: 3,058
Likes: 454
From: Wilbur by the Sea, FL
"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.
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.
Addict
Rennlist Member
Rennlist Member
Joined: May 2001
Posts: 3,534
Likes: 38
From: Cary NC
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!
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!