Brake Pedal Kick Back

[Gregg1100]

Would a knackered rear anti sink sphere cause bleeding troubles also ???

[Mandrake]

Would a knackered rear anti sink sphere cause bleeding troubles also ???

Yes, a ruptured anti-sink sphere can bubble a stream of nitrogen into the rear suspension lines which will find its way into the rear brakes. So if you believe yours could be stuffed and you have plans to replace it soon it could be a waste of time bleeding the brakes now as you'll need to do them again afterwards. Unless you don't mind doing it twice of course.

[Gregg1100]

How does one know when anti sink is naff, apart from bubbles in lhm, which you can't see till you try to bleed anchors, and how long do they last ?? Or is that a " how long is a piece of string" question ??
Before I took the car off the road nearly 2 years ago, I had 3 instances of having a hard brake pedal, and no stopping action--a quick up and down on the pedal resulted in full braking again. ABS light is working correctly---self test and off. What would that be, please ??
A regular question mark today - me.

[Mandrake]

How does one know when anti sink is naff, apart from bubbles in lhm, which you can't see till you try to bleed anchors,

Often there is no real clue that the anti-sink sphere is naff or nearly naff until it fails completely and you find yourself having to bleed "air" (really nitrogen) from the rear brakes every few weeks... even then a naff anti-sink sphere may not always give any symptoms at all.

It's real purpose is a brake accumulator for the real brakes, and it's there as a safety feature. To understand why its there and what it does a quick recap on older pre anti-sink Citroen brake systems.

In most of those (including GS and BX, and 1993 Xantia's) the front brakes are supplied via the brake control valve directly from the high pressure supply that comes from the pump. Since we don't want our brakes failing on us while driving due to a snapped axillary belt the accumulator sphere as well as smoothing out the pressure from the pump provides pressure storage that is able to operate the front brakes fully several dozen times before the pressure drops too low to be effective.

But what if the accumulator sphere was knackered and the axillary belt snapped ? You'd be in trouble, except that the rear brakes are supplied on a separate circuit with the pressure coming from the rear suspension instead of the pump/accumulator sphere. This has two cool benefits.

One is that the rear suspension pressure is higher with higher loads in the boot, therefore the maximum braking of the rear brakes is proportional to the load in the boot. Light load, light braking, no (easy) locking of the rear wheels, heavy load and the brakes are allowed to work more fully.

The other is that even if your accumulator sphere was dead, and your aux belt snapped, you'd loose your front brakes but still have normally functioning rear brakes (plus the hand brake on the front) since they draw their pressure from the rear suspension.

Fast forward to the anti-sink system from 1994 onwards. In this system when the SYSTEM pressure (at the main accumulator sphere) drops below the suspension pressure special isolation (anti-sink) valves kick in which isolate the suspension struts from both the height correctors, and in the case of the rear suspension the brake control valve. The height correctors and brake control valve are the most "leaky" components in the system that cause the suspension to go down quickly on older "sinker" models. With these isolated by a low leakage cut-off valve the car can stay up for days.

The problem is that this disconnects the rear suspension from the brake control valve, thus no rear brakes! If the main system pressure went away for some reason (naff accumulator and snapped aux belt) then you'd loose your rear brakes too.

To avoid this the anti-sink system adds an anti-sink sphere which is connected to the input of the brake control valve which feeds the rear brakes. Thus even if the anti-sink valve closes and isolates the rear suspension, you still have stored pressure in that anti-sink sphere to operate the rear brakes. The sphere should really be called "rear brake accumulator".

A lot of things have to go wrong to see a worst case scenario though. Anti-sink sphere and accumulator sphere both punctured and the aux belt snapping or pump failing. A faulty anti-sink sphere can unfortunately go unnoticed if everything else is ok, but its a good idea to replace it if its likely to be duff just for the peace of mind that you have that extra backup for your rear brakes should something else go wrong.

and how long do they last ?? Or is that a " how long is a piece of string" question ??

It is unfortunately. For suspension spheres I'd say their typical life before the ride starts to deteriorate too much is about 3 years, but they'll usually keep going about 5-6 years before they reach the point where they rupture, so I'd say an anti-sink sphere would be similar. If its older than about 5 years I'd definitely be replacing it.

Before I took the car off the road nearly 2 years ago, I had 3 instances of having a hard brake pedal, and no stopping action--a quick up and down on the pedal resulted in full braking again. ABS light is working correctly---self test and off. What would that be, please ??

Really hard to know with just those details, did the car have a known good accumulator sphere at the time ?

I'm assuming no warning lights came on the dash during the problem, and you say that it fixed itself within a second or so ?

My first thoughts would be the ABS - just because the self test light behaves as expected doesn't mean everything is right with it. Sensor faults can sometimes cause erratic behaviour even without setting a fault or leaving the warning light lit.

Were you going fast at the time or just pulling to a stop ? It's not uncommon for a faulty ABS wheel sensor to cause the brakes to completely (and scarily ) fail for a fraction of a second at low speeds (<10mph) if one of the sensor gaps is excessive or there is an intermittently broken wire. Sometimes you'll feel a pulsing on the pedal as you slow to a stop. By the time you'd release the pedal and press it again the momentary fault would have passed.

Basically the confusing/incorrect signal from one or more wheel sensors makes the ABS think the wheels are locking up so it turns off the brakes momentarily in an attempt to halt the "skid" that it has detected. In a fraction of a second the fault either passes or the ECU realises from checking the coherency of the signals from the different wheel sensors that something "doesn't quite add up" and re-enables the brakes. Small intermittent glitches aren't always enough to log a fault though.

ABS ECU's have a lot of failsafes programmed into them but they still aren't perfect, at the end of the day its still a computer that has the ability to override your decision to press the brake pedal and release the brakes!

[Gregg1100]

Simon- you are a STAR. I followed your fail safe bleeding instructions for obstinate Xantias. All the air was gone within 1/4 litre of fluid, same on the o/side.
Only problem is I will have to do it again. When I said the oil was green, I lied. . Turned out a crappy browny green lhm was more browny than green.
The MOT had an advisory on the brake hoses on front end, so will get them and a shed load of LHM to pump it through the system. I doubt I will do the 600 miles needed to clean system out, so will just change whats in there.
Thanks again,
Greg

[Gregg1100]

Brakes momentarily failed as I was about to go over a Mt. Everest size speed hump- so just crawling. I have personally always thought abs was iffy--especially in a 44 ton truck. Takes away the feel of the brakes, plus abs can't see what is happening in front of it. I know some here on this forum like it, I don't. And would never have a motorcycle with abs fitted.
Perhaps the air in the system was giving out a signal to abs, so will wait to see if clear now.

[Hell Razor5543]

What is the mileage on the Xantia? Citroen recommend replacing the LHM every 36,000 miles, and at every 108,000 to run a hydraflush cycle (to clean out any accumulated crud/silt). The fact that the LHM is brown says it needs replacing (but you already know that).

[Mandrake]

Simon- you are a STAR. I followed your fail safe bleeding instructions for obstinate Xantias. All the air was gone within 1/4 litre of fluid, same on the o/side.

Ah, excellent. I haven't known that method to fail yet, the trick is that the rear suspension must be carrying weight to guarentee that there will be sufficient pressure available to bleed the rear brakes. If the wheels are hanging limp there might or might not be depending on the adjustment of the height corrector...it doesn't take much of a sticky height corrector linkage or misadjustment of the manual override lever for the hanging wheels/axle stand method to fail, so I don't bother to even try it that way, I just do it the way I know will always work, even though you're only doing one wheel at a time with more jacking up and down involved.

Only problem is I will have to do it again. When I said the oil was green, I lied. . Turned out a crappy browny green lhm was more browny than green.

It could be that only the oil in the brake lines is browny green but the rest isn't as bad. If anyone has changed the oil before but hasn't given the brakes a good bleed afterwards the dead ends formed by the brake lines will still have the mucky old stuff in it. By the time you've bled about 1/4 to 1/2 a litre you should have flushed through the entire run of brake line, if the oil didn't get any cleaner then it's that dirty in the main circulating system as well...

Another good reason to use a short hose and a bottle instead of a long hose to the tank - it lets you see whether what comes out is fresh and clean or old and mucky before deciding whether to reuse it or not!

[Mandrake]

Brakes momentarily failed as I was about to go over a Mt. Everest size speed hump- so just crawling. I have personally always thought abs was iffy--especially in a 44 ton truck. Takes away the feel of the brakes, plus abs can't see what is happening in front of it. I know some here on this forum like it, I don't. And would never have a motorcycle with abs fitted.
Perhaps the air in the system was giving out a signal to abs, so will wait to see if clear now.

Yup, that sounds like the ABS alright. It will be a problem with the signal from one or more wheel sensors, its very unlikely to be the actual ECU.

Pinning down which sensor it is if its very sporadic could be challenging although a bit easier if you have access to a Lexia, it could be one of a number of things on any one of the wheels:

* Sensor tip to toothed ring gap too wide, this can happen if the sensor is removed and the heat shield is placed under the sensor as a spacer instead of on top of it. See the following thread for an example of that happening:

http://www.frenchcarforum.co.uk/forum/v ... =3&t=44613" onclick="window.open(this.href);return false;

The tip should be almost touching but not actually touching. (maybe the thickness of a couple of sheets of paper for a gap)

* The toothed ring has rusted away so there aren't clearly defined teeth anymore... I think it has to be pretty bad to cause trouble though, my front ones are both badly rusted and eaten away to the point where it looks like it shouldn't work, but it still does!

* Corrosion on one of the ABS connectors on the subframes. They're a bit of a pain to get to though... the rear ones are above the subframes to either side of the spare wheel carrier, the front ones are on the sub frame below and to the sides of the engine. A bit of contact cleaner on the connectors could help.

* Broken wire in one of the cables - the ABS sensor cables have to flex with suspension movement so if there is a tiny break in the wire they can be intermittent with suspension movement... one of the rear sensors on my previous Xantia had a slightly dodgy cable (I believe) because if I travelled over a large speed bump at around 5-10mph with the brakes applied as the rear suspension hit the bump the brake pedal would suddenly pulsate rapidly as if it was reacting to prevent a skid. (ABS should not operate below 10mph, if it does its a sign that a wheel speed signal is erroneous)

I never did get around to fixing it or confirming the exact cause as it never happened during normal driving. My current Xantia does not exhibit the same symptoms.

* The sensor itself could be faulty.

If you do have access to a Lexia you could try looking at the wheel speed readings on the live sensor data while someone drives very slowly or while you're spinning each wheel by hand to see if one sensor gives a lower reading than the others or suddenly drops to zero or becomes erratic at low speeds.

[Hell Razor5543]

It could be worth checking out the sensors using the diagrams in the following post, to see if any are going into high resistance. It doesn't take much to worry the ABS system;

http://www.frenchcarforum.co.uk/forum/v ... 11&t=26012" onclick="window.open(this.href);return false;

[Gregg1100]

To Simon and James,
I have changed the n/s/f sensor, ages ago- and the o/s/r in the last 2 years(not long before I took the car of the road). Fault was there just before it went off the road.. I have a copy of the ECU pins- I will have a look see tomorrow--if nothing found, I will wait for car to be back on the road and basically see if it happens again. Will report back.
I wish there was someone near me with a Lexia-- other faults could be sorted as well.

[CitroJim]

I wish there was someone near me with a Lexia-- other faults could be sorted as well.

Greg, would you like to be the first to borrow the Forum Lexia?

You'll be OK if you have a laptop running Windows XP.

I'm trying to source a laptop for it and then can look at doing loans of it properly... Watch this space...

[Gregg1100]

Hi Jim,
Thanks for your kind offer, but I wouldn't have a clue as to how to use it. Probably do more damage to car than sort it. On the old Cryptons of years ago, I could check the points gap with them, but that was all. No XP anywhere here either--all Win 7 Pro and Win 8 Pro. I wouldn't want the responsibility of having something like that here, in case it got dropped, made naff or nicked.
Thanks anyway,
Greg

[CitroJim]

No worries Greg... Hopefully someone can come to your rescue with a Lexia. There must be someone nearby?

[RichardW]

Gregg

Simon missed one.... As I said earlier in the thread check the front rings to see if one is broken. The end of the shaft corrodes and expands the ring till it breaks, this puts an effective extra tooth on the ring, and confuses the ABS - and the end result is the ABS activating when it shouldn't at low speeds. Mine go so bad one morning on the way to work it would hardly stop - I had to take the fuse out to get it to stop!