intermittent brake failure

[toby5959]

As a new member I thought I would give it a try in hopefully getting some help with a braking problem on my 53 reg xsara picasso 2.0 HDI. I drive about 30k a year and the car is about to go over 100k. Last month whilst crawling along in a queue, the outside temp showing 32C, went to stop but the brake pedal went straight to the floor, next push of the pedal and all worked well, and has done for the next 4000miles. Yesterday, outside temp showing 38C, travelling at 70mph a car pulled out infront of me, went to slow down but the brake pedal again went to the floor with absolutely no stopping power at all. Pushed on the brake pedal again and all was in working order.
My first thought was the brake fluid boiling, but I don't have to pump the brakes to get it back. It seems to me that maybe a vacuum pipe intermittently collapses, master cylinder intermittently fails or more likely some electronic part breaks down when hot. Suggestions would be most welcome as the car is sitting on the drive with a sad face not being taken out to play.

[Stewart(oily)]

Most likely to be the Master cylinder, ISTR having those symptoms with a failing check valve.

[citronut]

It seems to me that maybe a vacuum pipe intermittently collapses,

if this was the case the pedal would go solid not to the floor, as you would just loose the servo assistance,

the brake wouls still work but you'd have to press harder

[Mandrake]

Could be ABS related. Intermittent wheel speed sensors have been known to cause momentary complete loss of braking at very slow speeds around 5-10 mph. It usually only lasts for a fraction of a second so by the time you released the pedal and pressed again it would be working again.

I'm not so sure about whether it could happen at 70 mph though. Have you ever had the ABS system read for fault codes ? If it happens very rarely it could be a challenge to pin it down.

[toby5959]

ABS system fault codes are all clear. Not even thought about wheel speed sensors and like you unclear whether they can give a false reading at any speed. I have a niggling feeling that these high ambient temperatures that we are having at the moment may contribute as it only occurs when the tarmac is about to melt !!

[Mandrake]

The wheel sensors just generate a certain number of pulses per revolution which the ECU counts to determine the wheel speed.

The road speed is determined by the wheel(s) with the greatest rotational speed at a given instant, and any wheels deemed to be turning drastically slower than the fastest wheels (or not turning at all) when braking are assumed to be locking up and the brake for that wheel is momentarily released, with the ECU watching to make sure the rotational speed of the wheel increases again. (Consistency checks) Some cars tie the operation of the rear wheels together for ABS, but front is always independent.

So what happens if a wire to one of the sensors goes open circuit due to suspension movement flexing a broken cable ? As far as the ECU is concerned that wheel has just suddenly locked up as its no longer receiving pulses from it, so it will release the brake on that wheel.

However if after a certain time the wheel speed doesn't increase then the ECU will know that the lack of wheel speed signal is bogus because if the other wheels are doing 50mph and it released the brake on the wheel and it still says 0 mph half a second later it knows something is wrong and most likely the fault light will appear and it will start ignoring that wheel sensor.

There are a ton of consistency checks that the ECU does so they're not easily fooled, but a brief intermittent loss of connection to or short on the sensors triggered by suspension/cable movement can confuse the ECU momentarily.

On my previous Xantia every time the rear wheels went over a large enough judder bump at a slow speed of around 5-10mph if I had my foot on the brake as it hit the bump I could feel the pedal rapidly pulsate due to the ABS kicking in. (It never lit a fault light) I never did fix that but it was probably a broken wire to one of the rear sensors. My current Xantia doesn't do this.

[xantia_v6]

I don't think that ABS faults typically cause the pedal to go to the floor.
I would expect this to be a master cylinder fault.

[toby5959]

Thanks guys, you have been most helpful.

[spider]

I'd agree likely to be master cylinder too.

Faulty ABS rings (have seen this) result in lack of braking from front or rear usually but the pedal although moves further does not drop to the floor.

As pointed out a broken / collapsed vac hose to the servo (seems unlikely as large vac hoses usually have metal coils in to prevent internal collapse on depression ie vacuum) you'd end with a rock solid pedal although braking is still available albeit you'd have to press a bit harder.

I thought about collapsing flexi hoses but these tend to do a one-way valve trick (rare thankfully) which results in a brake sticking on rather than anything else.

Personally I'd replace the master cylinder from your symptoms, at least this way all the fluid will be changed too for sure.

[jetmir85]

Hi All
I am Having the same problem on my Xsara Picasso 2001 2.0Hdi.
i have changed the master cylinder and it does not change anything.. it is the same problem.
Does anyone solve this problem?

[Fizzeek]

Hello, my wife c3 has this very fault, how did you resolve this please ?

Jay

[GiveMeABreak]

That post is over 3 years old, so I wouldn't expect a reply any time soon.

If you have a specific issue, I suggest you create a new thread and describe the problem in as much detail as you can.