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Brake system complexity and safety

I have a 2005 Prius with 132K miles (bought it new). A couple of years ago all the brake warning lights came on. I parked the car for a couple of hours. When I restarted it, no warning lights. No problems since (30K miles). My mechanic scanned the codes and found that there was some sort of problem with the solenoid in the left, front brake.

On PriusChat I see quite a few problems with Prius brake systems. I suspect most are due to the complexity of the software that deals with anti-lock, anti-skid, regenerative braking, etc. Some people claim that they lose braking completely, which is obviously very dangerous.

Do the same problems appear in non-hybrid cars? Although I’ve been happy with my Prius, since non-hybrid cars are getting in the range of 40 MPG, I’m thinking that my next car should be less complex. I do miss shifting gears for myself, which is becoming a lost art. Safety is my main concern.

BTW, I’m a retired software engineer who has designed many software-controlled systems. So it may seem odd that I don’t completely trust them, but I know that all software of this complexity has bugs in it. Mechanical things wear out. Software either works or doesn’t.

"Software either works or doesn’t."

As a software engineer, I’m surprised you would make such a statement. Especially since you pretty much contradict it in the prior sentence saying you expect bugs in complex code. Surely you know that exercising every possible thread in every possible combination along with variable timing conditions makes “it either works or it doesn’t” a tall statement…

If you don’t trust modern technology, Don’t get on a modern airliner. A Boeing 777 has multiple miles of wiring in it, along with complicated software to run it all.

I agree that shifting gears is becoming a lost art.

Brake warning lights are common when a weak/discharged 12V battery sags during the startup cycle.

All modern cars with abs systems have solenoids, the prius does have a more complicated braking system, The prius system is measuring the stroke and pressure generated at the brake pedal and translating that into braking force created by a generator during regenerative braking. You can still have solenoid issues with abs units on non hybrid cars. I believe the solenoids are on the abs unit, not the actual wheel.

The biggest problem with prius friction brakes is they are not used enough, The majority of the time, the pads never reached conventional operating temperatures and
corrosion could occur between the backing plate and friction material due to the fact that the pads never dried out. On some vehicles, the corrosion between the friction material and backing plate would cause a complete separation.

The rear drum brakes on older priuses were bullet proof, like any drum braking system they are superior to disc brake setups. Disc brakes have a few advantages, But IMHO the next generation prius should have 4 wheel drum brakes, I have contacted toyota and gave them my feedback and they hopefully will pass it onto engineering.

Any car with 4 wheel disc brakes is going to have issues with rotors corroding and pulsation, it all comes from a basic design defect in disc brake setups.

Disc brakes are for Nascar, where they only need to work well for a short period of time. When it really counts, Count on our old friend the drum.

TwinTurbo, I stand corrected. My comments were incomplete and somewhat flippant. There are many examples of software being close enough (error in one pixel on my TV screen) and mechanical systems working or not (the seal of the lid when I can preserves).

I am somewhat cynical about companies spending the money to exercise “every possible thread in every possible combination along with variable timing conditions.” It seems like I hit a combination in the braking system on my Prius that had not been tested. Of course, it could have been something like a small particle in the brake line that caused a sensor to signal a problem, which eventually passed by the sensor. And I still wonder about the reports of brake failure that I see on PriusChat.

This still leaves the crux of my question: Has the hybrid technology taken us to a software (actually, firmware) complexity where companies do not test thoroughly enough? At least with a manual transmission I can downshift to at least slow my car down. And if the engine is racing, I can shift a manual into neutral, thus removing some burden from the parking brake.

WheresRick, yes, I have read that a weak 12V battery in the Prius can cause warning light issues. But I had driven my car about 50 miles. Then I left it parked for about an hour. Then I got in and drove it another 2 - 3 miles and had stopped at a traffic light when my problem arose. I had my foot on the brake pedal. It suddenly gave about an inch, and the warning light lit up. As I drove away (VERY carefully), I had to use MUCH higher pressure on the pedal to activate the brakes. I actually drove the car home before parking it. In addition to the clear lack of power braking, the handling of the car was very twitchy.

I doubt that it was the 12V battery because I have not changed it, and I’ve put another 30K miles on the car over the past 2 years with no further incident. (Crossing my fingers here.)

TwinTurbo, another example of mechanical systems exhibiting binary behavior. In 1969 a brake line on my 1965 SAAB sprung a leak. Fortunately, that car had a diagonal dual brake system, so I was able to stop my car, albeit with reduced braking power.

I was a student at the time and had just picked up a young woman hitchhiker. As I put on the brakes an pulled over to the side of the road, she asked in a very concerned voice, “What are you doing?” When I explained the situation, she agreed that I was doing the right thing.


“Count on our old friend the drum.”


On Friday, I started working on an Astro van. Those drums are so dependable, that they’re out of round. I’m cutting them on the lathe.

But you’re right about one thing . . . we can count on our old friend to provide us with job security. Replacing the shoes, wheel cylinders, cleaning up the backing plate, applying white lithium grease, replacing the hardware, adjusting the shoes, adjusting the cable, etc.


Of course you can pull the gear selector down on a automatic(try jamming a car in 1st at 80 mph)any way before you hit anything take the ditch or some nice dense brush(I still cant believe that people had runaway cars they couldnt stop) any complex system can develop glitches from time to time,stick with the Prius(I assure you they have been throughly tested-and hybrid cars will only get better)
@Rick,drums on trucks,discs on cars,please?(you will have to admit that drum brakes are very economical on trucks-have no trouble getting over 200K on backdumps in mountainous areas)-Kevin

Have you checked if your car is covered under the recall for the brake assembly? Under all the electronics is still a hydraulic brake system that will function with no power at all.

Wes, having driven drum brakes for a few decades before rotors became commonplace, I have to respectfully disagree on the superiority of drum brakes. In the old days it was very common for drum brakes to get wet in heavy rains and puddles, and since the drum captured the water it was terrible getting them to dry out again. “Riding the brakes” to dry them was the commonly accepted practice. Disc brakes have totally eliminated that problem.

While warped discs are certainly not uncommon today, warped drums were not uncommon many years ago. I’ve cracked one in the past too, another failure mode not unknown with drum brakes. Disc brakes, their frictional surfaces being exposed, cool far more readily than drums. Drums cracked from thermal shock (a hot drum going through a cold puddle) were another problem with drums that disappeared with discs.

You posted a good post, and I know we’ve debated this issue before, but I truly believe disc brakes are far superior to drum brakes. I know I may be opening up another tangential spin off to a main thread subject, but I had to comment.

I have had many more drum brakes than disk. They all worked OK at normal speeds. I am on my second vehicle with 4 wheel disc brakes. At over 5 years my pads are at 85%. I tend to be easy on brake pads and clutch discs. My problem with 4 wheel discs is the “baby drum” parking brake. They tend to be very ineffective compared to proper rear drum brakes. My KIA is OK pointed downhill but not OK pointed uphill. It was inspected and cables adjusted at the dealer without much improvement. If I have to park on a slope where there is no curb to turn my tires against I get the brick out of my trunk and chock a wheel.

This still leaves the crux of my question: Has the hybrid technology taken us to a software (actually, firmware) complexity where companies do not test thoroughly enough?


This is exactly the concern I posted about back when the first instances of braking anomalies were being discovered. I’ve been developing embedded, multi-tasking, real time firmware for safety and mission critical applications for ~ 40 years now. I wonder to what standard the automotive industry is held when developing their firmware…

thanks a lot mountainbike! when someone is being hardheaded you automatically assume its me? :slight_smile:

it was Rick. drum brakes, alcoholic attitude, remember?

Bang the drum [brake] slowly;
hijack the thread.

@TwinTurbo, yes, I know what you mean. I developed system software for a CT scanner back in the late 70s. We had a “deadman switch” on the X-ray source. If it didn’t get a start pulse, the hardware turned it off within (as I recall) one second. Of course it could have gotten caught in a loop, but at least a system freeze would cause the X-rays to shut down. The overhead crane system I worked on in the early 80s had huge mechanical stops at the ends of the track to stop the gondola if the software failed to do so. A developer was riding in the gondola one day while debugging the software. He said that although he knew about the mechanical stops, traveling full speed toward the water was still quite a thrill.

I would like to think that the hydraulic lines on my Prius brakes would still be working if all the computers in the car were fried. At least that was the feeling I got during the incident when all the brake warning lights lit up. But I would like to hear that from the engineers, not the marketing department!

Yes, the Prius braking system is complicated but, like all braking systems sold on cars in the US, there must be redundancy. If power assist is lost, a hydraulic line fails, the ABS/TCS/ESC fails, or the re-gen system fails you must still be able to stop the car. No single point of failure full brake loss is allowed. Requirements and testing for regular cars, EV’s or hybrids are spelled out in FMVSS 135. That doesn’t mean the brakes are not degraded, but they must still stop the car.

That said, as many have pointed out, complex systems and software can be problematic. manufacturer’s testing should include every single issue that might be encountered in the operation and failure of the car.

As for drum brakes, some folks just won’t admit when a new technology is superior. Nothing that many years of psychotherapy can’t cure!

“As for drum brakes, some folks just won’t admit when a new technology is superior. Nothing that many years of psychotherapy can’t cure!”

I am amazed at the number of people buying vinyl albums again because they think they “sound better”.


Interesting. I spent quite a few years working on CT/PET system for non-invasive brain cancer detection. My area of development was firmware to accept the host system RX and execute the various sub-systems; XRay generation, DAS, cradle and gantry operations then correlate and package up the data before sending back to the host. We were at 350ms/slice at full resolution. 1200lbs of gear and support structure at that rotation speed! With the covers off, it was quite impressive. Doubt many would put their head in there if they knew what it looked like beyond the flimsy plastic facade.

Your crane story reminded me of the control software snafu at a nuclear power plant. The crane had extracted the rod and was transporting it when a software glitch caused it to go the stops and sit there beating against it. Now what…


No single point of failure full brake loss is allowed

That would be a foregone conclusion. I would submit that any reasonable DFMEA on a modern system would conclude that full loss shouldn’t even be allowed with the majority of two point failure modes considering the severity level of the failure mode…

@David L., agreed. I did a side-by side when changing from vinyl to CD’s. To my ears the CD was superior.

I’ve had it explained to me by an electrical engineer that digital music (CD’s, mp3’s, transistor amps) has harmonics on the odd numbers, 1,3,5,7 and analog music (tube amps, vinyl records) have harmonics on the even numbers 2,4,6,8 giving a “warmer” more pleasing sound. Since all speakers are essentially analog except for active crossovers, the other stuff is what matters. Maybe you’d need a turntable hooked to a tube amp to get the full effect?

@mustangman: I’m not sure what the electrical engineer meant by that. Both systems have all the harmonics present. The “warmer” sound that vinyl enthusiasts claim is the result of distortion and compression, not odd or even harmonics. More odd harmonics generally give a more “nasal” tone (oboes have more odd harmonics and are closer to a sawtooth wave; flutes have less harmonics and are closer to a pure sine wave).