Xantia brakes having a 'wobble'

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andmcit
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Xantia brakes having a 'wobble'

Post by andmcit » 26 Jan 2006, 19:35

Mate in work has bought himself a TD Xantia 1.9 SX saloon on a P plate and is happy with his new charriot.

One thing the worries him a bit about it however, is a tendency for the brakes to replicate a kind of slow motion ABS effect under ANY braking from high or slow speed. The symptom is felt as a on/off/on/off etc biting of the brakes although there is NO foot pedal bounce youd expect with air in the system, AND THE CAR PULLS STRAIGHT under heavy braking. He hasn't ACTUALLY tried to provoke the brakes to cut the ABS in as he doesn't fancy buggering up his tyres if it all does lock up...

The ABS light does the normal routine in start-up and stays OFF ALL THE TIME THE CAR DRIVES although I wonder just exactly how 'handy' the previous owner was with his maintenence of this test function...

but maybe that's just me being a bit cynical.

For what it's worth, the car has a fresh out the oven new type MOT cert and seems a nice tight sorted car other than this braking issue.

I've suggested he takes the time to get the circuit bled and do a check of the calipers and discs etc to see if anything untoward is wrong. He mentions an occasional high pitched squeel but cannot determ ine if it's from the front or back.

Only other suggestion I've made to him is tio find a Cit Indy who may manage to converse in Citroen gobbledeegook with the diagnostics to see what fault codes land out the machine.

Has anyone come across this one before? I'd be interested in the remedy of this one myself!

Andrew
Last edited by andmcit on 27 Jan 2006, 09:20, edited 1 time in total.

mezuk04
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Xantia brakes having a 'wobble'

Post by mezuk04 » 26 Jan 2006, 19:36

Let him know that its a Hatchback and not a saloon :wink:

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Mandrake
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Re: Xantia brakes having a 'wobble'

Post by Mandrake » 26 Jan 2006, 20:35

andmcit wrote:Mate in work has bought himself a TD Xantia 1.9 SX saloon on a P plate and is happy with his new charriot.

One thing the worries him a bit about it however, is a tendency for the brakes to replicate a kind of slow motion ABS effect under ANY braking from high or slow speed. The symptom is felt as a on/off/on/off etc biting of the brakes although there is NO foot pedal bounce youd expect with air in the system, AND THE CAR PULLS STRAIGHT under heavy braking. He hasn't ACTUALLY tried to provoke the brakes to cut the ABS in as he doesn't fancy buggering up his tyres if it all does lock up...

The ABS light does the normal routine in start-up and stays OFF ALL THE TIME THE CAR DRIVES although I wonder just exactly how 'handy' the previous owner was with his maintenence of this test function...

but maybe that's just me being a bit synical.
Testing if its the ABS misbehaving is easy - simply remove the ABS fuse from the fuse box (the ABS light should stay on on the dashboard) and take the car for a drive. The brakes should behave EXACTLY as a model with no ABS - eg normal braking but you will be able to lock the wheels.

I think you'll find its not the ABS though.

What it sounds like to me is a dud accumulator sphere, and since you dont mention it I presume it hasn't been checked. With the accumulator sphere very flat every time you apply the brakes it will cause a large pressure drop, and then the pump will cut in and you'll get a large pressure rise - the accumulator isn't able to smooth the pressure changes.
For what it's worth, the car has a fresh out the oven new type MOT cert and seems a nice tight sorted car other than this braking issue.

I've suggested he takes the time to get the circuit bled and do a check of the calipers and discs etc to see if anything untoward is wrong. He mentions an occasional high pitched squeel but cannot determ ine if it's from the front or back.
Could be the auxillary belt squealing slightly when the hydraulic pump load comes on ? If the accumulator is nearly flat or punctured it puts a heavy load on the pump. The belt could be a bit loose too of course.

Regards,
Simon

john alexander
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Xantia brakes having a 'wobble'

Post by john alexander » 26 Jan 2006, 21:05

Hi its none of the above, its the spring inside the tube fitted under the brake pedal . As you brake the spring compresses and causes the pedal to pulse under your foot ,you will get used to it but its better to replace it with a piece of steel tube. There are many postings on here ,some with pictures but basically lie in the footwell,lift up the brake pedal, theres a black plactic tube fitted into it. pull it out ,it may be tight ,I had to use a screwdriver. Slide the collar up take out the two half circle pieces and the tube comes apart, there's a spring inside ,replace it with a piece of tube the same length,rebuild the plastic assembly and re fit it . Warning the brakes are now super sensitive, mind your face on the windscreen. I think most people on this forum have done this mod.

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Mandrake
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Xantia brakes having a 'wobble'

Post by Mandrake » 26 Jan 2006, 21:15

Oops, I completely forgot about the pedal spring mod :oops: :oops:

All 3 of the Xantia's in our family have had this mod done and I take it for granted now so it didnt even occur to me to suggest it.... :)

Search for a thread called "Terrible Xantia Brakes" by DoubleChevron, and perform the mod.

I would still advise checking the accumulator sphere and the auxillary belt tension as well however.

Regards,
Simon

andmcit
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Xantia brakes having a 'wobble'

Post by andmcit » 26 Jan 2006, 22:51

john alexander wrote:Hi its none of the above, its the spring inside the tube fitted under the brake pedal . As you brake the spring compresses and causes the pedal to pulse under your foot ,you will get used to it but its better to replace it with a piece of steel tube. There are many postings on here ,some with pictures but basically lie in the footwell,lift up the brake pedal, theres a black plactic tube fitted into it. pull it out ,it may be tight ,I had to use a screwdriver. Slide the collar up take out the two half circle pieces and the tube comes apart, there's a spring inside ,replace it with a piece of tube the same length,rebuild the plastic assembly and re fit it . Warning the brakes are now super sensitive, mind your face on the windscreen. I think most people on this forum have done this mod.
Just so that I'm totally clear I fully get this spring damper business explained to myself before I speak to my mate! I get what you've explained elsewhere...

Yes, OK there's this bloody stupid spring to sanitise and dumb down the PROPER Citroen lightswitch braking effect that you'd naturally expect as par for the course on a CX/DS/GS brake valve. A whole load of extra slack in the system so Mondeo driver's don't have to 'suffer' any culture shock driving a Citroen! [sounds a bit like too much Peugeot influence if you ask me!! :wink: ]

A good thing to know, and a useful mod I'll do on my Xantiae when time permits.

BUT how will this spring in the pedal assembly make an effect that I've described where for all the world it's like the brake pads 'pulse' on and then let go off and on again [and so on] under ANY braking effort? NOT the pedal pulsing - the whole car under braking!!

I've been told the pedal on this Xantia we're talking about doesn't have any kickback or pulse like you'd get on rubbery air-in-the-hydraulics brakes such as I've encountered on duff motors in the past!!

Surely what the spring in the pedal assembly WILL create is slack until it's taken up and acting onto the button on the brake valve, whereupon the pressure will/should be consistent?

Am I being dense and missing something here!?

Andrew

andmcit
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Re: Xantia brakes having a 'wobble'

Post by andmcit » 26 Jan 2006, 22:59

Mandrake wrote:
andmcit wrote:Mate in work has bought himself a TD Xantia 1.9 SX saloon on a P plate and is happy with his new charriot.

One thing the worries him a bit about it however, is a tendency for the brakes to replicate a kind of slow motion ABS effect under ANY braking from high or slow speed. The symptom is felt as a on/off/on/off etc biting of the brakes although there is NO foot pedal bounce youd expect with air in the system, AND THE CAR PULLS STRAIGHT under heavy braking. He hasn't ACTUALLY tried to provoke the brakes to cut the ABS in as he doesn't fancy buggering up his tyres if it all does lock up...

The ABS light does the normal routine in start-up and stays OFF ALL THE TIME THE CAR DRIVES although I wonder just exactly how 'handy' the previous owner was with his maintenence of this test function...

but maybe that's just me being a bit synical.
Testing if its the ABS misbehaving is easy - simply remove the ABS fuse from the fuse box (the ABS light should stay on on the dashboard) and take the car for a drive. The brakes should behave EXACTLY as a model with no ABS - eg normal braking but you will be able to lock the wheels.

I think you'll find its not the ABS though.

What it sounds like to me is a dud accumulator sphere, and since you dont mention it I presume it hasn't been checked. With the accumulator sphere very flat every time you apply the brakes it will cause a large pressure drop, and then the pump will cut in and you'll get a large pressure rise - the accumulator isn't able to smooth the pressure changes.
For what it's worth, the car has a fresh out the oven new type MOT cert and seems a nice tight sorted car other than this braking issue.

I've suggested he takes the time to get the circuit bled and do a check of the calipers and discs etc to see if anything untoward is wrong. He mentions an occasional high pitched squeel but cannot determ ine if it's from the front or back.
Could be the auxillary belt squealing slightly when the hydraulic pump load comes on ? If the accumulator is nearly flat or punctured it puts a heavy load on the pump. The belt could be a bit loose too of course.

Regards,
Simon
The car has just recently had a full set of new spheres replaced on it by someone I've been told should know what they're doing. The accumulator sounds healthy with it's cycling duration/timing too.

Andrew

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Mandrake
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Xantia brakes having a 'wobble'

Post by Mandrake » 26 Jan 2006, 23:16

Hi Andrew,

There is some debate about the exact mechanism by which the spring introduces the "pulsing" effect on the brakes (have a read through that "Terrible Xantia brakes" thread and you'll see "heated" discussion about it :) ) but suffice to say that it does happen.

I think its partly a human interface thing - the pedal travel vs pressure becomes very non-linear when the spring fitted (there is also a rubber snubber inside the spring, plus the spring bottoms out after a certain amount of pedal force) meaning that it becomes very hard to judge the pedal pressure/movement correctly.

The human brain seems to like a pedal that is either completely pressure sensitive (like a normal earlier hydropneumatic Citroen) or a pedal that is displacement sensitive. (A normal vaccum servo system)

With the spring you get some kind of bizare hybrid between the two where the relationship between pedal movement and pressure is changing all over the place through the range of operation.

What tends to happen is that moderate braking can't be controlled accurately so you end up over and under compensating, coupled together with the lack of feedback from the pedal due to the spring.

I think friction in the sliding of the plastic shell that the spring is contained within also plays a part in the inability to control the braking precisely.

The simple answer is "just do it" and see if it solves the issue. It's a very easy mod that can be done literally in 5 minutes if you have a suitable piece of tubing, and the mod can also be un-done just as easily by putting the spring back in...

Regards,
Simon

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Kowalski
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Xantia brakes having a 'wobble'

Post by Kowalski » 27 Jan 2006, 00:27

I agreee pretty much with what has been said above about the accumulator sphere being the first port of call, have a search on the forum for how to test it. If it isn't that sphere, it can only really be air in the system or a fault with the pressure regulator.

I know we've had this argument before but...

On my '97 I can feel through the brake pedal (with spring fitted) when I pull on the handbrake. After I changed the brake pads I could feel the adusters in the calipers adjust when I pressed on the pedal. The spring removes non of the sensitivity of the braking system on a Xantia.

Can somebody explain why my two Xantias BOTH with pedal springs as standard don't have pulsing brake pedals if thats the cause? My theory is that the pulsing isn't caused by the spring in the canister. As far as I'm concerned having the canister there or not is purely a matter of taste, if there is something wrong with the spring cannister fitted to a particular Xantia that isn't a design flaw, thats wear and tear.

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AndersDK
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Xantia brakes having a 'wobble'

Post by AndersDK » 27 Jan 2006, 02:22

Agree w/ Kowalski here.

You dont go modifying to solve a problem - instead you find the cause of the problem. Pulsating Xantia brakes is a wellknown symptom but its not caused by design.
Since all spheres have been replaced the system has been "opened" and allowed for air to get in all over.

Therefore I prescribe first a good portion of Citaerobics and then a complete brakes bleeding procedure.

BTW : there is an "emergency trick" to bleed rear brakes if rear bleed screws are knackered.

jeremy
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Xantia brakes having a 'wobble'

Post by jeremy » 27 Jan 2006, 10:49

Anders - an old trick I used to use many years ago in the days of Morris Oxfords and the like with knackered bleed nipples was to loosen the metal or flexi brake pipe going into the cylinder or caliper and bleed through there. With car it would work everytime and those connections didn't seem to corrode as well as the nipples. Vanny sheared one of the front of his BX the other day before he set off on his long run collecting bits, and he cleared the air from his front brakes using this trick.

Village
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Xantia brakes having a 'wobble'

Post by Village » 27 Jan 2006, 14:35

Am I being a complete divvy here? wasn't it stated that there was NO abs effect (the buzz) through the brake pedal? but it was the brakes that were buzzing (you know what I mean)NOT the pedal.

I'm confused (and thankful that my ZX has "normal" brakes and no ABS!!)

andmcit
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Xantia brakes having a 'wobble'

Post by andmcit » 27 Jan 2006, 15:05

Village wrote:Am I being a complete divvy here? wasn't it stated that there was NO abs effect (the buzz) through the brake pedal? but it was the brakes that were buzzing (you know what I mean)NOT the pedal.

I'm confused (and thankful that my ZX has "normal" brakes and no ABS!!)
Glad that's been noticed! :wink:

The calipers seem to pulse on off under braking NOT the brake PEDAL!! :D

Andrew

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CitroJim
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Xantia brakes having a 'wobble'

Post by CitroJim » 27 Jan 2006, 18:24

Try a really thorough bleed of all of the brakes, noting that at the rear you need to bleed a LOT of LHM out to ensure all the air is out.

I had exactly this on mine. That and the brake pedal spring mod (highly recommended) did the trick.

Also, clean the LHM filter in the reservoir and change the LHM if you have any doubt of the age of it. A hydraflush would not be a bad idea too.

jeremy
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Xantia brakes having a 'wobble'

Post by jeremy » 27 Jan 2006, 18:49

If you search this forum you will find that just about the only problem anyone has with the ABS system is that the warning light stays on and that this is generally a sensor problem. Malfunctions of the valving system are very rare, but some Xantias have suffered from ECU problems - but again this shows up on the check light. No mention has been made of the light not functioning properly - and if it wasn't the ABS would shut down in any event.

Xantias with anti-sink (like this one have a very small pump output for the suspension and brakes. this is perfectly adequate with a good accumulator but without an accumulator the only high pressure LHM for the brakes is supplied direct from the pump - so instead of having about 1/2 a litre of high pressure LHM available the thing is trying to make do on direct output of 2 cylinders of the pump. The cylinders themselves have an output of less than 1 cc - and so I'm sure you can see that things are far from ideal and its possible that the pump pulses can be felt through the pedal valve as well as the rest of the system.

The back brakes are of course different, relying on the pressure in the back suspension for their power.

If the accumulator has failed its likely that gas has been discharged into the system - which may be eliminated on doing 'Citroebics' (raise fully, drop, repeat cycle many times - gets fluid circulating) - and some may have found its way into the limbs leading to the brake calipers and will have to be bled out. As however they are one way only its possible that it hasn't got in there.