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Suspension & Braking
Have some questions about suspension or brake setups for your E46 BMW? Get all your answers here!

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Old 04-21-2018, 04:28 AM   #1
Viperbcs
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Complete brake failure while driving

Hi guys, did not have a lot of time reasesrching so please excuse if this is in the forum.

Dad lives in South Africa me in Europe and I normally service the cars when there once a year. He drives a 330i e46 04 model. Yesterday coming to a red light he lost brakes had to run a red light at 80km/h while turning avoiding traffic then stopped with the hand brake/park brake. DSC and brake light burning. Switched of the car and back on and all has been fine since then. I am seriously worried about this. Any idea or similar experience? Pedal went soft so i suspect a seal in master cylinder or booster.
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Old 04-21-2018, 04:35 AM   #2
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Pedal went soft makes me think a tear in the line somewhere or the master cylinder
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Old 04-21-2018, 04:43 AM   #3
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Suspect something like that as well, but pressure is fine since. Drove 30kms home
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Old 04-21-2018, 04:47 AM   #4
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Originally Posted by Viperbcs View Post
Suspect something like that as well, but pressure is fine since. Drove 30kms home
If it was the brake booster, wouldn't the pedal become hard and not soft?


Possibly air in the lines or the brakes faded?

Last edited by armenh7; 04-21-2018 at 04:47 AM.
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Old 04-21-2018, 06:37 AM   #5
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Booster alone then hard. The soft to the floor makes me think air in system(but how) or a seal in master cylinder
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Old 04-21-2018, 09:20 AM   #6
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Iím betting the master cylinder is bad. Itís not hard job replacing it, but verrrry difficult when youíre thousands of miles away!

The booster doesnít regulate brake pedal travel. Thatís done by the master cylinder. Do you flush the brake fluid when you service the car? That should be done every 2 years.
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Old 04-21-2018, 10:24 AM   #7
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Master Cyinder is where you start. It's the only thing in the system that can send the pedal to the floor and then recover and operate correctly. It will fail again and repeat the symptom.

The typical failure of the master cylinder is that the pedal sinks while holding the car during a traffic light. The pedal falls as you wait, you have to raise your foot and pump the pedal a couple of times to build pressure, As you hold the pedal down, it sinks again.
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Old 04-21-2018, 10:25 AM   #8
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Complete brake failure on a modern vehicle is pretty uncommon. This is due to the split design and the dual chamber master cylinders.

Either something like oil has been put in the master cylinder or reservoir and contaminated the brake fluid, there is a stuck caliper that caused brake fluid to boil or the is some other basic problem that should be pretty obvious.

Power brake booster failures do not a loss of brakes, unless the driver does not push extremely hard on the brake pedal. The pedal will be FIRM and will not go to the floor, but often people are caught off guard and do not apply the brake pedal with enough force and "think" the brakes have failed. Power brake boosters do not often fail immediately and do not recover if they start to fail and they do not trigger any warning lights when they fail.

The ABS systems are overlay devices, you could disconnect every electrical connector on a ABS system and the car should work like and old school late 1960's or early 1970's vehicle.

It appears that the DSC and Brake warning lights may be on? If this is the case, these are clues that something may be wrong with the braking system.
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Old 04-21-2018, 04:42 PM   #9
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Thank you all, will keep you posted. Dad took it to the local mechanic and he advised to leave the car there(for obvious reasons). He will check codes but suspect a seal in the Master cylinder.

Brake fluid was flushed and there were no warning sign before this happened. Car now has 232,000 km on the clock(around 145,000miles)
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Old 04-21-2018, 04:47 PM   #10
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jfoj nailed it- stuck caliper AND probably internally leaking master.

That's about the only way a dual circuit system will totally fail.

t
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Old 04-21-2018, 07:37 PM   #11
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Originally Posted by TobyB View Post
jfoj nailed it- stuck caliper AND probably internally leaking master.

That's about the only way a dual circuit system will totally fail.

t
But how does the DME know the stuck caliper or leaky MC to display DSC and brake light?

The computer controlled the ABS and it could releases all calipers pressure if it wrongly determined a locking brake event. Turning off the engine (reset the computer) seemed to fix the problem. I say the electronic at fault here.
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Old 04-21-2018, 10:25 PM   #12
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The abs computer monitors pressures and pedal position.
It's quite possible that it saw something out of range, and lit up.

While I agree an electronic failure's not impossible,
BMW (at least in 2002) was horrifically paranoid about
that, and unless abs went into ice mode or something
else odd that we haven't seen since the E30,
the electronics shut themselves OUT of the loop if they
go outside their normal parameters.

The classic failure is that the master cylinder fails- at some point-
so that both circuits are connected, and the system no longer has redundancy.
It still works, but when a sticky caliper boils the fluid in one wheel
or another, now the fluid vapor (usually water) displaces a bunch of
fluid, and now the entire system is useless. The offending caliper cools,
the driver's seat is cleaned out, and suddenly, brakes are back. USUALLY a bit
more spongy, but 80% of their former selves.

It's also not too unusual for an aggressive braker to lose just the front circuit to a cooked caliper,
and go right on through the intersection because the rears alone don't have a chance in hell of hauling
the car down from 80 kph to a stop in 40 meters. And the pedal's drop to NEAR the floor
is interpreted as a complete failure.

But that's all internet conjecture. Someone with some ability will have to diagnose
the thing, and that'll take having a car in front of them.

Mental note- next time I bleed the brakes, stomp on them with one
circuit open, and see if there's still the other circuit there...

t

Last edited by TobyB; 04-21-2018 at 10:27 PM.
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Old 04-22-2018, 03:14 AM   #13
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But all this theory doesn't explain why DSC and brake light were ON.
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Old 04-22-2018, 08:05 AM   #14
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OP, how much brake fluid is in the reservoir?
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Old 04-22-2018, 08:53 AM   #15
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Originally Posted by Sapote View Post
But all this theory doesn't explain why DSC and brake light were ON.
Quit with the "THEORY" you are not a Theoretical Physicists, might play one on TV.

Get a tool/software to READ the ABS related codes AFTER someone checks for proper fluid level in the master cylinder reservoir.

Once codes for ABS are read, then some sense of what is going on can be discussed once facts and information is in hand.

ABS cannot really determine there is a stuck caliper. A stuck/dragging caliper can cause the brake pad and fluid to over head. Brake fluid can and will BOIL, contaminated brake fluid will boil at a lower temperature than the non contaminated brake fluid. . Once the brake fluid boils, it compresses, because as most understand liquids do not compress, but gases do compress.

As mentioned, once boiling brake fluid cools off, it pretty much goes back to a "normal" state of a non compressible fluid until it boils again.

A possibility is the car could have been operating with 1/2 of the redundant braking system already in a failure mode, then the 1/2 of the system that was actually working failed.

As for the ABS system, as I mentioned, it is an overlay system, unless the electronics are damaged in a way, possibly due to moisture, that bleeder solenoids were opened when they should not be to relieve braking pressure, I have a hard time understanding or believing there was a failure in the ABS system that caused a loss of braking.

There is clearly more to this story, nobody is hands on with the car so everything is 100% pure speculation at this point. The poster is not the owner, driver or even in the same continent as the vehicle, so more factual details will need to be gathered.
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Old 04-22-2018, 10:03 AM   #16
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I've seen it happen over the years although not recently in the newer cars. It was always a master cylinder. Anything else would have other consequences and not completely recover on its own. Replace the master cylinder. On a wackier note I'm not sure we are even allowed to call them master cylinders anymore in our crazy society.
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Old 04-22-2018, 03:37 PM   #17
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theyíve been making cars for decades now with a split braking system for safety. meaning one line operates the front Right & rear Left while the other line works the front Left & rear Right. so if one line breaks the other works a front/rear brake.

no brakes at all suggests the master cylinder that supplies both most logical spot to check.

if one line was bad and car was only using half the braking system almost any driver would be able to notice odd braking prior to overheating fully and having no brakes or pedal feel.
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Old 04-22-2018, 07:48 PM   #18
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I've seen it happen over the years although not recently in the newer cars. It was always a master cylinder. Anything else would have other consequences and not completely recover on its own. Replace the master cylinder. On a wackier note I'm not sure we are woven allowed to call them master cylinders anymore in our crazy society.

Agree that a bad MC could do this, but why the DSC and brake lights were ON?
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Old 04-22-2018, 08:33 PM   #19
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Agree that a bad MC could do this, but why the DSC and brake lights were ON?
When someone get software or a scan tool that supports ABS, then they will know what the errors are and likely be able to determine what the reason is.

But also a simple brake fluid check is also the place to start for all of this.

Until then, nobody knows.
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Old 04-22-2018, 08:42 PM   #20
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But all this theory doesn't explain why DSC and brake light were ON.
Yes, it does. If there's vapor in one circuit, circuit system pressures will be very mismatched = error.

If there's vapor in one circuit, and the abs pump tries to add pressure, it will not be able to = error.

Lots of other scenarios possible, but as we keep saying, without the car in front of someone, it's all internet fluff.

t
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