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Old 01-22-2017, 04:24 PM   #1
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Unhappy Failure to engage brakes when hard pressed

My son had an accident and says the pedal just went straight through with no effect but worked just before and after so he won't drive the car until he can afford to get a mechanic to look at it but he needs the car to work so I am hoping someone on here might be able to point me to the answer.

He noticed that the brakes were extra touchy when he started driving that day but were working so proceeded to drive, stopping at two stop signs and braking for two turns without failure. After the second stop sign he pulled into the right lane of a four lane road with dividing turn lane, signaled to change to the left lane and waited. When an opening occurred he merged to the left only to have the car in front slam on the brakes because the light had changed. He tried to slam on his but the pedal went straight to the floor and he had to swerve. The impact stopped the car. He had slipped under the bumper of the SUV in front of him impacting only his passenger side headlight area so the car can be made driveable but we need to be sure it is safe.

Does the touchiness of the brakes prior to the accident give any clue to the failure when he slammed the pedal? Is there a possibility that the a separate system caused the failure under extreme pressure but doesn't stop the brakes from working the rest of the time? Is it safe to drive?

Thanks for any help you can give. We are taking it to a Saab mechanic in a couple of weeks but don't have an alternate vehicle for him to use before that.
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Old 01-22-2017, 05:45 PM   #2
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I'd check his cell phone and FaceCrap account for texts/posts around the time he rear-ended the accident victim and take his keys for lying and not leaving enough space.

To more accurately diagnose the true root cause of the problem, did your son receive many participation trophies when he was younger and did you buy the car for him for free?

Just go in a big parking lot or a long straight-away, accelerate, and jam on the brakes. If they work they work.
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Old 01-22-2017, 05:56 PM   #3
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I'd check his cell phone and FaceCrap account for texts/posts around the time he rear-ended the accident victim and take his keys for lying and not leaving enough space.

Just go in a big parking lot or a long straight-away, accelerate, and jam on the brakes. If they work they work.
I understand your thought, but he wasn't on his phone or otherwise distracted, per his passenger and the other driver, and made no error that was visible to her or his passenger. The only possible error, which would not have been visible, was that he could have been pressing both gas and brake without realizing it through his heavy soled boot. However, I am still looking for a possible answer for the experience he related. He isn't a teenager, he is an adult.
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Old 01-22-2017, 06:00 PM   #4
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I understand your thought, but he wasn't on his phone or otherwise distracted, per his passenger and the other driver, and made no error that was visible to her or his passenger. The only possible error, which would not have been visible, was that he could have been pressing both gas and brake without realizing it through his heavy soled boot. However, I am still looking for a possible answer for the experience he related. He isn't a teenager, he is an adult.

To more accurately diagnose the true root cause of the problem, did your son receive many participation trophies when he was younger and did you buy the car for him for free?
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Old 01-22-2017, 06:03 PM   #5
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If the brake "failure" doesn't repeat, it was probably driver error, like you said. If he's an adult, he knows if he hit the gas and the brake at the same time or not. Have you trained him to recognize what anti-lock brakes feel like when activating?

I'm giving you a hard time, yes, but for real you should take it out someplace safe and just see what it does in a similar situation. He may not know what anti-lock activating feels like, and just exceeded the braking capabilities for the surface he was on...or, if there is a brake system problem it very likely should repeat, and then you know.
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Old 01-22-2017, 06:23 PM   #6
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Here is another way to ask the question: Is there any possibility that at a low speed the ABS system can malfunction and think the wheels are locked, thereby releasing the brakes long enough to allow the collision? He applied the brakes no less than 3/4 of a car length from the other vehicle and had enough room to go from slightly to the right of center on the other vehicle to almost clearing it on the left, so he couldn't have been too close.
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Old 01-22-2017, 06:28 PM   #7
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If the brake "failure" doesn't repeat, it was probably driver error, like you said. If he's an adult, he knows if he hit the gas and the brake at the same time or not. Have you trained him to recognize what anti-lock brakes feel like when activating?

I'm giving you a hard time, yes, but for real you should take it out someplace safe and just see what it does in a similar situation. He may not know what anti-lock activating feels like, and just exceeded the braking capabilities for the surface he was on...or, if there is a brake system problem it very likely should repeat, and then you know.
Hey, no problem, my husband and I would have been the same if my husband hadn't talked to the other driver and the passenger.

He hasn't had a license very long and might have misinterpreted the ABS feeling as a failure. I am hesitant to take the car for a test drive until the fender is moved back forward a little as I just had new tires put on and don't want to ruin them if they rub the fender but I'll have to take it up and down the street a couple times and see what I can find out. Thanks for the suggestion, I hadn't asked him if he let up when he felt it 'go through' or kept applying it while he steered to the left.
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Old 01-22-2017, 06:43 PM   #8
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If you did a FMEA, I think the more likely failure mode would be loss or decrease of vacuum assist--there are some check valves and such involved with that system and their failure would increase the pedal pressure required. If a check valve in the crank vent system sticks open, or hose is moving under load or something weird, you could see variable effort on the pedal, the brakes would still work though. It would just require more pedal effort.

If the ABS were malfunctioning, there is some amount of built-in-test that would set the ABS malfunction lamp. So that's one layer of assurance that the electronics are functioning correctly,

Beyond that, if the ABS lamp's not on, and if you check that the brakes actually work a few times after you get the fender fixed, then I'd feel pretty comfortable that they're good. In industry and for critical systems like nuke plant and airplane controls, there's actually a name for doing that, it's called a function check or validation check, which mathematically acts to 'reset the clock' on system Availability calculations to some extent.

By calculation the Availability, or probability that a system will be functional on-demand, of a mechanical system typically decreases with age, but an end-to-end test or proof check that specifically validates the proper function of a system boosts those numbers for a period, even if each component of that system may not have been individually tested.

Is the car automatic or manual? The autos have an additional cam driven vacuum pump that the manuals don't have so that adds a few extra potential failure points in the overall system. Also, does the car stall or run rough at idle? Those things indicate vacuum issues.

So, long story short, if the car brakes normally a few times when you test it, and the ABS lamp's not on, then the problem may be inside the interior sitting in the driver's seat. Leaving more space never hurts, even if there is a malfunction, extra space can save you.
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Old 01-22-2017, 07:12 PM   #9
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If you did a FMEA, I think the more likely failure mode would be loss or decrease of vacuum assist--there are some check valves and such involved with that system and their failure would increase the pedal pressure required. If a check valve in the crank vent system sticks open, or hose is moving under load or something weird, you could see variable effort on the pedal, the brakes would still work though. It would just require more pedal effort.

If the ABS were malfunctioning, there is some amount of built-in-test that would set the ABS malfunction lamp. So that's one layer of assurance that the electronics are functioning correctly,

Beyond that, if the ABS lamp's not on, and if you check that the brakes actually work a few times after you get the fender fixed, then I'd feel pretty comfortable that they're good. In industry and for critical systems like nuke plant and airplane controls, there's actually a name for doing that, it's called a function check or validation check, which mathematically acts to 'reset the clock' on system Availability calculations to some extent.

By calculation the Availability, or probability that a system will be functional on-demand, of a mechanical system typically decreases with age, but an end-to-end test or proof check that specifically validates the proper function of a system boosts those numbers for a period, even if each component of that system may not have been individually tested.

Is the car automatic or manual? The autos have an additional cam driven vacuum pump that the manuals don't have so that adds a few extra potential failure points in the overall system. Also, does the car stall or run rough at idle? Those things indicate vacuum issues.

So, long story short, if the car brakes normally a few times when you test it, and the ABS lamp's not on, then the problem may be inside the interior sitting in the driver's seat. Leaving more space never hurts, even if there is a malfunction, extra space can save you.
It is an automatic. He says he did not let off the pedal, which was doing the ABS jump, but admits that there is a small possibility that his boot caught the gas pedal. The car runs perfectly smooth so I doubt there is a vacuum problem unless it is outside of the parts that can affect the engine performance, but it does have a 'service engine soon' light on.
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Old 01-22-2017, 07:34 PM   #10
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Give it a test once it's back together, and go ahead and take it somewhere and ask them to check the vacuum lines and other brake and brake booster connections. I wouldn't bother with that second part if it were my car and the brakes worked, but he may need that to feel better, and who knows? Maybe there is something that can be found? That said, maybe being a little concerned that the car won't stop in the distance that he thinks it should isn't such a bad thing either.

Otherwise, you might consider that unless you grow up doing donuts in the snow and burnouts at stop lights and other socially unacceptable things like that on purpose, most people don't really know the performance envelop of their vehicles--especially if you throw some water or snow into the equation.

I've taken a lot of people to an empty parking lot and had them go full wide open throttle and pretty fast, down to 0 as fast as possible a few times to see what it feels like and learn what's possible and not. Some people are reluctant to even press the brake hard enough to activate the ABS and truly don't know how fast their car can or can't stop.


Anyway, good luck!
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Old 01-22-2017, 08:03 PM   #11
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Thank you very much for your suggestions! I will take it out and bury the brakes a few times to satisfy myself that it was most likely driver error before I insist he drive it. Even so, I think we will take that approach on the wide open space training, if I can find one around here, and throw in a few lessons on how to use the hand brake just in case it was an intermittent system failure.

My son isn't the dare-devil type but my husband was, as you describe, one of those donut and burnout sorts so I'll leave the extreme driving lesson to him. I'm also going to buy my son some "driving shoes" instead of always wearing his jack boots.
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Old 01-22-2017, 08:09 PM   #12
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To more accurately diagnose the true root cause of the problem, did your son receive many participation trophies when he was younger and did you buy the car for him for free?
No participation trophies, barely even got him to ride a bicycle let alone drive anything.

Yes we bought the car for him "for free" to save his job but intend to sell it to him rather than give it to him so he isn't off the hook for the money we are spending on it and that drives him crazy because he hates being in debt. This is one of the reasons it may take some strong encouragement to get him back behind the wheel. But I don't want to push him back 'on the horse' without making sure it wasn't the horses fault. Thanks again for your help with that.
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