Sudden brake failure in mk3 VW Golf

Ian Johnston ( snipped-for-privacy@btinternet.com) gurgled happily, sounding much like they were saying :

I bought a GSA with only one rear disk _fitted_ a few years ago...

You don't seem to be taking the op's previously demonstrated driving skills into account. Tell her to switch off the ignition and I wouldn't be at all surprised if she removed the key and put it in her handbag, then looked up to see where she'd got to. At which point, she makes a small correction to the steering, it locks at an angle and she's heading for the central reservation...

Oh FFS, you thought I was being serious?

Do you know much about the general workings of a car?

Turning the engine off but leaving the car in gear will not only give excellent engine braking, but in the miraculous event of the OPs brakes suddenly working again, there'd be more than a little vacuum, meaning the servo would assist well enough.

Though anyone who finds their brakes aren't working _several hundred yards_ before impact and still manages to shunt into the traffic ahead needs to not have a licence. Obviously the OP figured it was better to write off as many cars as possible through her inabilities, than just her own.

Oh FFS, you thought I was being serious?

Yeah, we all know about the vacuum left in the servo yadda, yadda, yadda (well done to you for pointing out the stark staringly obvious we all know and didn't feel we needed to tell each other), but here is no point turning off ignition and not a good idea for the average "just-about-able-to-drive" motorist. Many, but not all cars can end up with the steering lock on (we have had this debate before, not again - please) and then there is the heavy steering and as I said lack of servo if brakes recover. First gear will brake you safely to a few miles an hour and its not good advise to turn off engine. The thinking here is totally black and white. The brakes have stopped working, therefore they have permanently failed (obviously not true as the police found no mechanical problems with the car). In my experience, nowt is certain and its quiet likely the brakes would recover and far from "miraculous". If my brakes failed the last think I would want is the controls suddenly becomming heavy and unfamiliar by turning off the ignition, just at the moment I need all the help I can get.

However, I strongly suspect the OP is of the school that you just press harder if they don't stop you, no pumping or easing off pedal in case its a wheel slid she didn't realise was happening. These days a car would have to be a massive pile of junk to have brake failure. I drive around in 15-20 year old vehicles and never had any hint of the car or van not stopping in

30 years. Even some serious piles of rust I drove in my teens never hinted at failing. My guess is she was slidding and the "few hundred years" is not an accurate assessment of the distance.

-- bucket

But if you're in gear you'll still have servo brake assist & powre steering, whatever position the keys in. Unless the throttles jammed open though you'll get negligibly more engine braking.

Not what ive been informed but I'm willing to be swayed.

Im willing to be swayed on the idea because I'm not working from first hand knowledge (despite THRASHING my cars at times ive never boiled brakes).

So you're saying the water remains as steam or that you bled it there and then before it had chance to...

??

Also if theres water in there there will be oxygen in it (the water), just like tap water. Boil tap water and no matter how much you cool the steam back to water you will have released oxygen that cannot be forced back into it without special methods. This is how fish run out of oxygen in stangnant pools - the oxygen leaves the still water and doesnt re-absorb.

I'm talking reasonable quality fluid replaced fairly often, even the wet boiling point of DOT3 is up near toasted discs/pads territory IIRC, certainly DOT4.

Coyoteboy ( snipped-for-privacy@hotmail.com) gurgled happily, sounding much like they were saying :

In that case, it was at the end of a motorway slip road. I'd *just* managed to stop before the roundabout through judicious use of pumping the pedal like mad and handbrake.

Out with the 8mm spanner, up with the bonnet, and crack the nipple open (the joys of inboard front brakes!)... Exeunt steam...

Right, let's go back a step... Glycol DOT brake fluid is hygroscopic. As it's exposed to the atmosphere in a braking system, it absorbs water from the atmosphere. That lowers the boiling point of the fluid, because the water absorbed in it boils and turns into steam. Liquids are not compressible. Steam is a gas, and therefore IS compressible, hence no brakes. As the steam cools, it condenses back into water which is reabsorbed by the glycol fluid.

If you boil the pond, I'd suggest that the oxygen is irrelevant to the survival of the fish in the pond.

LHM is non-hygroscopic, unlike the hygroscopic DOT3/4 crap that most of the industry insists on using for some bizarre reason. However, when the master cylinder reservoir has been submerged and has mud in it, that's theoretical...

Hygroscopic brake fluid is used for that reason, so that if any water does get in, it'll be absorbed and gradually lower the boiling point of the fluid.

DOT5 fluid was going to be the next big thing, until people actually stopped to think about the inherent risk should water get into it. With DOT5, the water would make it's way to the bottom of the system (ie. the brake calipers/cylinders), whereby instead of having some fluid that boiled at a couple hundred degrees (or gradually lowering as the water content increases), you've got some fluid that boils at a hundred. And then you've also got the issue of the water freezing in a non hygro-scopic system.

I know that LHM is different from DOT5, but the same principles apply.

I was suggesting that unless you literally stopped that second (as it seems you did)and tested it you wouldnt get steam by your own admission. I dont need the process of water absorbtion and steam creation explained - I passed GCSE chemisty :)

My point being that if you boiled the water in the lines you would get some oxygen released, which is in turn compressible and would not re- absorb - so you would have a spongey pedal (to some extent, greater or lesser).

The situation where you have non-hydroscopic brake fluid and push water/mud into the master cyl wouldnt it take a long time for the water to get near the calipers to turn to steam, and only if the water is heavier than the brake fluid?

There is also the possibility - which I think we should at least consider

- that the OP was driving without due care and attention, rammed a line of cars without making any attempt to use perfectly good brkaes, and is trying out her court defence here.

Ian

I disagree. If the water absorbed the oxygen naturally in the first place, it'll damn well do it again when placed under high pressure in a brake system.

Usual method for making water absorb oxygen is to give a large surface area of contact and ensure good mixing occurs. Dont see that happening in a brake system, but of course to some extent it will occur given long enough.

Since it is a slightly exothermic operation the high heat of the brake fluid would tend to reduce the reabsorption further.

=============================== There's also a strong probability that the OP doesn't really exist. Her name, 'moriarte' closely resembles Sherlock Holmes's villainous adversary, 'Moriarty'. Her first post suggests that she actually used a form of pulsed braking (including some use of the handbrake) over 'several hundred yards' without any success. She also had time whilst covering that distance to verify several times that she wasn't pressing the wrong pedal- confirmed by no rise in engine revs. And the police found no mechanical failure despite the fact that the OP had been able to floor the pedal several times without any sensation of braking during the time she was travelling through urban traffic busy enough to cause a 'queue of traffic' at a set of traffic lights.

All very improbable, I think.

Cic.

In some cases the front. In some others the rear. In some others, both.

Both. And the relevance of this to the OP as she tries to stop for the lights is what exactly??

Steve

The braking effort is biased towards the front, to an amount approximately equal to the weight on the front end as a percentage of the vehicles weight. This is why there are load sensing valves on vehicles, less weight on the rear, a lower proportion of the braking effort is applied to the rear.

No. but it is more likely to cause a spin than the opposite scenario.

No, I'm not that stupid.

I do normal driving, like hopefully the OP.

A handbrake turn is vastly different to the OP's problem, remember her? She's trying to stop at the lights with no front braking and pulling on the handbrake which operates the rear wheels on the golf. With that drag on the rear wheels and no braking on the front, she is not going to go into a spin.

A handbrake turn is deliberately induced, and involves higher speeds than hopefully the OP was doing, also it involves extreme steering input which the OP is unlikely to have been doing. Different animal entirely.

Steve