Need help with brakes seizing/locking up at very slow speeds (under 10 mph) most commonly just before coming to a complete stop. This started about 2-months ago and became so bad that we finally got an OnStar alert with a error code reference code C1100 indicating the issue was in the ABS Module or associated circuit. We took the car to a mechanic friend who connected to their computer to confirm the C1100 and he unplugged the ABS module and then plugged the module back in again and reset the error code. This seemed to correct the problem for the last several weeks but the problem has returned but only occasionally. I have posted this problem on several forums but apparently this is very unusual since very few experts have been able to help me with what to do to remedy this issue. I hesitate to take to a dealership for fear of expensive ordeal.unless there is some way to pin point what the problem is. Our mechanic friend kept the car for 3-days and 300 miles and was never able to get the problem the duplicate. The error code C1100 has not returned.

Can anyone help??????

To start with, I'm suspecting your vacuum booster, but first, some Q&A:

Are they actually "seizing/locking up"? Or are they just going from [predictable braking response] to [extreme braking response] ?

For diagnostic purposes, let me frame this as [OnStar gave an error code C1100 indicating where it believes it saw the issue] . . . important because it might point to something that was affected by the issue, but not necessarily to the smoking gun.

Good, and hopefully your mechanic friend verified the condition of the ABS module, pump, and wiring while doing this . . . ?

Reading below I infer that "the problem" that returned occasionally is the last-10MPH-severe-braking problem and not the C1100 code, right?

Hopefully we can get a leg up on those other forums by solving this one. They'll be sooOOoOo jealous!

It is helpful that you have mechanic friend as a resource.

Verifying: "The error code C1100 has not returned" ever at all since the one time it appeared when OnStar told you about it, right?

Hopefully we can!

Okay, so here'* what I see: Severe-braking problem degrades until the event is so severe that the computer notices a bad reading, and since OnStar watches the computer it lets you know.

Above I was talking about the smoking gun. Keep in mind that the computer reports what it sees, but not necessarily the at-fault part or nearby parts. For example: PCM reports dead guy laying in the OK Corral. That'* all it knows. Did he have a heart attack? Is a lightning storm rolling through? Has there been a stampede? Is there a guy holstering his piece twenty paces away? The PCM doesn't know any of these things. It just knows <uh-oh dead guy>.

Back to your car: I've seen a few brake boosters illicit this type of behavior on older cars. Stuff is better now I know, but brake boosters are fundamentally very similar to how they've been for half a century, which only means you have to be even luckier than "1968 you" in order to have one fail. The units that I've seen failed like this were on non-antilock cars. Essentially they'd go full-assist as if you are suddenly standing on the brake pedal, and if you were very very careful on the brake pedal they'd not lock up until the last few miles per hour. On those barges, that meant the front wheels did a lot of braking, and the rears tried too, but since the car is now standing on its nose the rear tires get unloaded and lock up. I still own one of those cars today.

I suppose a person could test this possibility by having a mechanic friend plumb a vacuum gauge into the vacuum-supply hose going towards the booster, and watch the gauge'* behavior during normal driving and during the problem. I've never done that but I think it would useful information. I'd expect that under normal braking the gauge wouldn't move around much, and during an "event" the gauge would go to near zero vacuum then return in a few seconds. Maybe one of the others can chime in here.

Either way, let us know how it turns out and/or more information and/or current status. It'* how we learn stuff and sometimes make the other forums green with envy.

Thanks for your response CatherdralCub ….it'* a lot!!!!

1. Yes, when the issue occurs the brakes actually lock/freeze up ….one has to hit the gas to break free. Not sure how one measures between "predictable" and "extreme" as you termed it. Since this customarily happens at slow speeds I don't think it is a dangerous situation but really annoying.
2. Regarding the OnStar alert with the Code C1100, OnStar actually sent me an email regarding the Antilock Braking System with the "red sop sign" indictor that calls for Immediate Attention. Up until we received this notification we had no idea what the problem was but it had been going on for a week to 10 days before this and seemed to be getting worse. Keep in mind this wasn't happening all the time but just occasionally …..for example, my wife and daughter normally use this car and had been complaining about it until one day I took the car to the Walmart parking lot to just drive around slowly and was able to get it to repeat the locking/freezing four to five times. Here'* what someone (an expert?) on another forum posted trying to help "Here’* a link to what C1100 means. It’* either an ABS pump switch or the wiring to the switch."
3. The "mechanic friend" is an import mechanic who normally works on BMW, Audi, VW, etc. who did not have the capability to verify the condition of the ABS Module, Pump or Wiring. He used his diagnostic computer to connect into the on board computer to very the existence of the C1100 error code. From his experience with imports he unplugged the ABS module and re-plugged it back in attempting toe clean and reconnect the needle pin connectors which he indicated this sometime worked on the imports that he works on. And, this seemed to work for the next 4-5 weeks where we dove the car without experiencing the locking/freezing situation until just 2 weeks ago.
4. Two weeks ago our daughter was using the car for several days and experienced the locking/freezing situation again for 4 or 5 times. But the C1100 Code has not returned ….,,,,we received the OnStar monthly report yesterday and there is no reference to the C1100 code. An expert on another forum wrote that he though the issue was driver related here'* what he stated "When the driver stops too fast, and one or more wheels begin to lock up, the ABS system takes over. It sees the wheel or wheels slowing down too rapidly by way of wheel speed sensors. When this happens, it shuts off brake fluid to the wheel with the issue.".
5. Following the experience referenced in 4. above, this is when we took the car back to our mechanic friend where he keep the car for several days and drove it for 300 miles ……..this was all to see if he was able to experience the freezing/seizing that my daughter had recently experienced. However, he was not able to get the problem to duplicate and I have been driving it for the last 4 or 5 days without a problem. So, this is why I am hesitating to take the car to the dealership for repairs until I am able to identify what kind of a problem we have.
6. You are the first to mention that you suspect the problem may be a vacuumbooster issue. But you mention that this is usually an issue with older vehicles and are pre antilock brake vintage vehicles. On another forum someone suggested that I purchase a infared remote temperature reader to record the temperature at each wheel or determine if there'* a caliper or brake line issue. Another expert suggest that the problem is with the wheel bearings or the hubs. Perhaps that'* the reason that I am stating that the problem is likely an ABS Module system issue since that'* the only hint that I have had from a diagnostic standpoint, i.e., OnStar reported the C1100 error code, the ABS warning light on the dash has never come on ….not once.

I know this is a lot, but if anyone has any advice please let me know.

Isn't there a sensor that monitors the vac in the booster?

The thing that bugs me is that this only happens at low speeds.....hard to believe there is not a TSB on this....

My thoughts as I go below. This is a weird issue so whatever I throuw out there may not be a final diagnosis etc..

You're welcome . . . and yeah I do tend to get wordy.

"hit the gas to break free" tells me that something is dragging or locked "on" like a caliper or maybe the parking brake. You haven't mentioned that the brakes pull towards the left or right so whatever might be locked on is symmetrical. Have you had any work done on this car in the last few months before this issue started?

"predictable" is "I press the brake pedal lightly and the car slows gradually, I press moderately and the car slows moderately, etc." . "extreme" is I press the pedal lightly or moderately and the car attempts to merge my face with the steering whee using the inertia of my head.

Brakes will sometimes "grab" like this, only at low speeds, when a wheel cylinder is leaking brake fluid onto the braking surface. Seems like a long shot in your case, but worth mentioning. This doesn't fit well with the "hit the gas to break free" symptom though. To look at this possibility, check for any wetness around/on all calipers, rotors, etc. Also check brake fluid level.

Yeah, I think anything they alert you related to brakes will say same. I'd bet that is a requirement from the lawyers.

Assuming "four to five times" out of more that five applications of the brakes in the Walmart parking lot, right?

Might be, I just dislike throwing parts at something without a real diagnosis that points right at that part.

Foreign or domestic, many systems and parts are similar. He still saw it, and dis/reconnected it, and I assume never said "oh my goodness, look at how dreadful that looks!" in the process, right?

Good move, I would have done this as well. A thought: have him do this again and see if the issue goes away again. This could greatly narrow the search for a diagnosis.

The description is loosely correct, however, you've personally driven the car at low speeds in the Walmart parking lot and had the issue. A brake system doesn't remember how it was driven earlier today or last week, except in the case of extreme use where it will "remember" in the form of glazed brake pads or warped rotors or etc. Also, the ABS system won't continue to "hold" the car in place after it has stopped, requiring one to "hit the gas to break free". I'd discount this for now.

Makes sense to me. A question: what has been done to the brake system by anybody? Rotors, pads, calipers, etc.? I know it is new enough to possibly not have had any of these done, but good to know if it has.

A gut feeling, but yeah I still suspect the vacuum booster. I've had the issue on a few older vehicles, however, the technology is nearly the same today as on those. It'* not like the vacuum booster was reinvented twenty years ago and now is unrecognizable. If you've ever felt the brake pedal without the booster helping, you know it is happy to push really hard for you. Fortunately it does so under precise control when it works correctly. The issue I've seen on the older cars is when it is taken "off the leash" so to speak. It pushes hard because I gave it permission to push at all.

Think of it this way: The pedal pushes the piston in the master cylinder with the assistance of the vacuum booster. Because of this the master cylinder pushes brake fluid toward the brake calipers. This causes the calipers to apply pressure to push the brake pads against the rotors. The ABS pump is inserted between the master cylinder and the calipers so that (when commanded) is rapidly fluctuates the fluid pressure as needed to reduce locked brakes.

ABS or not, if the master cylinder decides to unleash its full available assist towards the master cylinder, the vehicle will experience a hard stop. The difference with ABS is that if this hard stop causes a wheel to start locking, it will attempt to correct.

Coincidentally (or not), engine vacuum supply is highest at idle, like when driving around the Walmart parking lot.

This is a good idea. After the car has been sitting all night, take it out on a long flat piece of road for like ten miles. Do your best to coast to a stop (using brakes as little as possible). Measure the temperature at each wheel. The front temperatures should be about the same as one another. The rear temperatures should be about the same as one another. Front versus rear should have the front a little higher. Now drive a little farther then brake to a stop like you would for a stop sign on a highway road. Measure again. Should have same results as described above, except at higher actual temperatures. In all cases, if one is significantly higher than the other it is either dragging, or the other isn't doing as much work as it should.

This is a long shot. If a wheel bearing caused the car to stop with such force then it would be making the whole interior growl and it would be audibly shrieking all the time. You'd be driving around looking embarassed at all the stares you'd be getting. The hub and the bearing are integrated on all four corners. The remainder of the hub (beyond the bearing) is three bolts and a bunch of steel. It'* pretty much impossible to damage a hub without damaging the bearing and vehicle at the same time.

I agree it is good to keep this in mind, but at the same time it is good to remember the computer only reports what it sees, and can't see everything. Since mechanic friend removed the issue for weeks by replugging the ABS module connector, I'd venture to say it might be wiring or the module, but both would be guesses.

Yeah, it measures vacuum versus decrease of vehicle speed, and the computer will alarm on falling widely outside that ratio. It'* an extremely-rare occurrence.

That'* a big part of why my gut thinks "vacuum booster". Low speed equals high vacuum, and if high vacuum gets fed incorrectly within the booster, a whole lot of assist can be applied to the MC.

I can't find any cases of this happening on this platform on the interwebs. Most of the promising leads go straight to other issues.

Someone on another forum mentioned disconnecting the battery for 30 minutes to see if that resets everything, similar to what some suggest when the check engine light comes on.

I have not done for fear of what else may be disconnected that I'm not aware of or know how to restore the unknown to previous settings. Can anyone speak to how feasible disconnecting the battery for 30 minutes might be?

Let me address some of the other questions that have been poised by others;

1. No,when the lockup/freeze occurs it doesn't pull left or right. Perhaps a better term for this condition is to term it as the brakes are "grabbing" instead of locking up/freezing as I termed this previously. Speeds have never been fast enough to throw a person into the windshield but it is uncomfortable in the car when the brakes "grab" and this seems to be relieved when one hits the gas pedal. Keep in mind that I personally have not experienced this for the last month or so since my mechanic friend unplugged the ABS Module and reset the C1100 code. My daughter is the only one who has experienced the brake grabbing two weeks ago following the ABS Module unplugging.
2. Calipers, rotors and brake fluid levels were all checked at the time of the ABS Module unplugging event. Everything checked out okay, in fact the pads still have over 50% wear left. My first reaction to the brake grabbing was a concern about the caliper slides or pistons being gummed up not allowing the calipers the release from the rotor engagement as normal.
3. The only work that has been performed, other than the aforementioned ABS Module unplugging, has been done at the dealership on March 23, 2018 when the car was taken in for a routine oil change and servicing including a tire rotation. The dealer is Bob Howard Buick/GMC, Edmond, OK and they are the only business that has ever preformed any work on the car. However, this has customarily only been oil changes and service work, nothing major has really gone wrong with this car. The car is due an oil change at 36,200 about 600 miles from now. Not withstanding a major reoccurrence of a brake grabbing event before next oil change I plan to have the dealership do a diagnostic check of the ABS Antilock System including the module, wiring and everything in that circuit to see if they can pinpoint where the failure was, or is, occurring. I am hoping that the cost for doing this is reasonable since the car is out of warranty in terms of time not miles.

Let me know about the advisability of disconnecting the battery for 30 minutes. That'* easy and won't take very long to do so long as I'm not creating a whole lot of other problems with having to go back a reset other things that disabled by disconnecting the battery.

All the 30 minute procedure does, is reset the modules in the vehicle.....it fixes nothing....

If you can not create the problem for the dealership, you will just be wasting your money.......basically, they will check for codes....do a TSB search, and then do a visual inspection.....if no codes, no TSB'*, nothing noticed from a visual inspection, and if they can't recreate the problem, nothing will be done.....

I would do nothing until the symptom becomes more frequent......then you can take a tech with you while driving and try to recreate the "concern"......the tech should have his Tech II installed and in snapshot mode....if the problem occurs, the tech can plot suspicious data on a computer screen....this is about the only way of fixing this problem, unless it just starts to do it all the time....

This is for those of you that assisted me with this issue, i.e., brakes grabbing at times.

Have appointment to take the Enclave in to the local dealership next Tuesday at 9:00 AM, Dealer indicates they have a recorder that can be installed on car that can be button activated when there are incidents where the brake grab or lock up to hopefully pin point the cause for this problem. They seem to acknowledge that there is an electrical problem somewhere in the ABS system that is causing these false alarms.

Good news is that the problem is likely covered as a drive train problem that is still under warranty since the car mileage is only 35,000 in spite of the age of the car which is 2013 model.

Again, thanks for your help and ideas.