Xantia MK II - Low rear brakes efficiency

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lancia58
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Xantia MK II - Low rear brakes efficiency

Post by lancia58 » 08 Jul 2019, 14:16

Hi

I failed the MOT today. The failure is low efficiency of the rear brakes ( 26%) . The rear spheres are almost "flat" ( very harsh riding ) from the rear brakes. Can the "flat" rear sphere be the cause for the low efficiency of the rear brakes ?

The front brakes efficiency is OK.

Its an automatic 2000cc 16v MK II ( Make year 2000 )


Thanks

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Mandrake
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Re: Xantia MK II - Low rear brakes efficiency

Post by Mandrake » 08 Jul 2019, 15:31

It's unlikely that low gas pressure in the rear suspension spheres will cause the rear brakes to be that poor, although you should certainly try to remedy that as well.

I'd be checking the condition of the pads and discs. My Xantia was an almost fail on the rear brakes last year despite the usual "clean up" of pads and surfaces etc, I've replaced the discs and pads a few weeks ago as the discs were a bit ratty and it will in for it's MOT shortly, so it will be interesting to see how the rear brakes test this time - they seem to work well in driving.

Another common problem on the Xantia rear brakes is that corrosion builds up and pushes the caliper out of alignment with the disc - have a close look and make sure the calipers are are parallel with the disc. If not there are plenty of threads on here about how to fix that!

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Re: Xantia MK II - Low rear brakes efficiency

Post by lancia58 » 08 Jul 2019, 16:24

I changed the rear disks and pads two years ago and since than I drove the car no more than 2000 km. I know that the mechanics bleed the system but I am not sure he did it correctly. If the system was not bled correctly can it cause this problem ?
Can you send a link to a thread that describes how to fix the misalignment between the calipers are are parallel and the disc ?

Thanks
Zohar

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Re: Xantia MK II - Low rear brakes efficiency

Post by Gibbo2286 » 08 Jul 2019, 18:54

If the brakes are not bled correctly you would know immediately, the pedal would be low and need pumping to get a proper firm feel to it.

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Re: Xantia MK II - Low rear brakes efficiency

Post by xantia_v6 » 08 Jul 2019, 19:15

Gibbo2286 wrote:
08 Jul 2019, 18:54
If the brakes are not bled correctly you would know immediately, the pedal would be low and need pumping to get a proper firm feel to it.

Thats not true on a Xantia, as it has high pressure hydraulic brakes. On a Xantia, air in the rear brakes primarily gives a delay in braking action when the pedal is pressed, maybe half a second or so, enough that the back-end of the car jumps up and then settles down again.

Poor brake efficiency on a Xantia is usually caused by either corrosion on the calliper mounts (causing the pads to be working at an angle), the wrong grade of pads (the correct ones have rather soft friction material), or a seized calliper piston.

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Re: Xantia MK II - Low rear brakes efficiency

Post by Gibbo2286 » 09 Jul 2019, 08:20

xantia_v6 wrote:
08 Jul 2019, 19:15
Gibbo2286 wrote:
08 Jul 2019, 18:54
If the brakes are not bled correctly you would know immediately, the pedal would be low and need pumping to get a proper firm feel to it.

Thats not true on a Xantia, as it has high pressure hydraulic brakes. On a Xantia, air in the rear brakes primarily gives a delay in braking action when the pedal is pressed, maybe half a second or so, enough that the back-end of the car jumps up and then settles down again.

Poor brake efficiency on a Xantia is usually caused by either corrosion on the calliper mounts (causing the pads to be working at an angle), the wrong grade of pads (the correct ones have rather soft friction material), or a seized calliper piston.


Old age must be catching up, :shock: forgot all about the Citroen system and I've run them for nigh on forty years.

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Re: Xantia MK II - Low rear brakes efficiency

Post by lancia58 » 09 Jul 2019, 09:51

Hi,

I changed the rear spheres and bled rear brakes, there was air in the barking system. For doing this I jacked the car's rear and set it the suspension to high. I noticed that when the car was jacked and height lever in normal riding position the rear wheels spin free even if I press the brake pedal. When the height lever is set to the high position and brake pedal is pressed rear wheels were locked . Is that normal ?

Thanks

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Re: Xantia MK II - Low rear brakes efficiency

Post by Hell Razor5543 » 09 Jul 2019, 09:54

I wonder if it is just that the rear brakes are (in part) fed from the rear suspension, and if there is no (or little) weight in the back the rear brakes don't NEED to work hard, so apply minimal braking force. I have known Xantia owners to turn up at the MoT with a couple of paving slabs in the boot, to make the rear brakes work harder. I had this happen once on one of my Xantia, but fortunately we were able to get a couple of people to sit in the back (myself and another MoT tester; the site was prepared to listen), and the extra weight got the rear brakes working harder, and she then passed.

I would check that the LHM fluid is at the fight level. If my memory is correct, what you should do is to raise the suspension to its full height, wait a minute or so to allow things to settle down, and then check that the ORANGE disc (not the brass one) is sitting between the two red lines on the bubble on the reservoir. If it is below them you need to top up the LHM, as this could allow air to be drawn into the system.

Something that could be worth a try (once you are happy that the brakes have been properly bled and should be working correctly) is a Citrobics session. This is a workout for the hydraulics, and can help free up sticking parts. Done properly it can take 20 minutes, but does make sure that everything is moving freely (as well as getting fully lubricated by the LHM). You will need the engine running and the handbrake on throughout the session.

Step one; turn the steering from lock to lock, waiting for a minute each time to allow the fluid to settle down before turning it to opposite lock. Do this a few times.

Step two; raise the suspension to its maximum height, and wait for a minute (again, to allow the fluid to settle down).

Step three; pressing hard on the brake pedal, move the suspension to its' lowest setting. The nose should drop at a normal rate, but the rear should remain high. After a minute or so (or when the leg cramps get painful!) release the brake pedal and hang on! The back should now fall like a stone. Wait again for a minute.

Step four; again, pressing hard on the brake pedal, move the suspension to its maximum height setting. The nose should lift at the normal rate, but the rear should remain down. After a minute or so release the brake pedal and hang on again! The back will attempt to launch itself into space! Wait again for a minute.

Step five; repeat steps three and four a few times, until you are happy everything is moving freely or you start to feel seasick!

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Re: Xantia MK II - Low rear brakes efficiency

Post by Mandrake » 09 Jul 2019, 10:48

lancia58 wrote:
09 Jul 2019, 09:51
I changed the rear spheres and bled rear brakes, there was air in the barking system. For doing this I jacked the car's rear and set it the suspension to high. I noticed that when the car was jacked and height lever in normal riding position the rear wheels spin free even if I press the brake pedal. When the height lever is set to the high position and brake pedal is pressed rear wheels were locked . Is that normal ?

Yes.

The rear brakes run entirely from the rear suspension pressure. If the height lever is in the normal position and the car is jacked up so the rear wheels hang down the rear suspension will be fully depressurised and there will be no rear brake pressure. Always set the height lever to maximum when trying to bleed the rear brakes.

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Re: Xantia MK II - Low rear brakes efficiency

Post by lancia58 » 09 Jul 2019, 12:30

What is the functionality of the central rear sphere ?

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Re: Xantia MK II - Low rear brakes efficiency

Post by Hell Razor5543 » 09 Jul 2019, 12:43

The accumulator sphere (and the mis-named anti-sink sphere; it is actually another accumulator sphere) are there to provide a pressure reserve should you lose the pump. It is not much, but should be enough to stop the car twice from motorways speeds should the pump fail.

Be advised that the brakes (barring the hand brake) on a Xantia are POWERED, and NOT power assisted. When you lose power and pressure you will lose the foot brake.

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Re: Xantia MK II - Low rear brakes efficiency

Post by GiveMeABreak » 09 Jul 2019, 12:46

Depending on your model. one will be for the anti sink and the other will be an accumulator sphere for hydractive systems.
Suspension Spheres.PNG
Suspension Spheres.PNG (48.79 KiB) Viewed 56 times
(1) Main pressure accumulator.
(2) Suspension sphere (front).
(3) Suspension sphere (rear).
(4) Hydractive suspension regulator accumulator (front).
(5) Hydractive suspension regulator accumulator (rear).
(6) SC/MAC storage battery. (Anti Sink System)
(7) SC.CAR accumulator. (Vehicles with Active Roll Control Systems only)
(8) SC.CAR regulator accumulator. (Vehicles with Active Roll Control Systems only)

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Re: Xantia MK II - Low rear brakes efficiency

Post by Mandrake » 09 Jul 2019, 13:29

lancia58 wrote:
09 Jul 2019, 12:30
What is the functionality of the central rear sphere ?

If you mean the one with the pipe coming directly out of the sphere, this is the rear brake accumulator sphere often called the anti-sink sphere as it first appeared in anti-sink models.

On older Citroen's the rear suspension pressure was permanently connected to the brake doseur valve. In the event that main system pressure was catastrophically lost the front brakes would fail (except in models like the CX with an additional front brake accumulator sphere fed through a one way valve dedicated to the front brakes) however the rear brakes would continue to work by the pressure stored in the rear suspension.

Anti-sink introduced in 1994 added anti-sink valves between the height corrector/brake doseur valve and suspension struts. When the main system pressure drops below suspension pressure these valves close and isolate the suspension struts from the height correctors and brake doseur valve - this prevents the car from sinking quickly, this is because both the height corrector and brake doseur are internally leaky by design and the anti-sink valves isolate these leaky devices.

However this introduces a major safety problem - if the main system pressure disappeared due to a catastrophic failure the anti-sink valves would immediately close, isolating the rear suspension struts from the brake doseur valve, and you would lose your rear brakes as well - no redundancy.

So a rear brake accumulator sphere was added - this is connected on the brake doseur valve side of the anti-sink valve. When the system is operating normally it is effectively connected in parallel with the rear suspension spheres however as it is piped through a long small diameter pipe it does not contribute significantly to ride. It is kept at the same average pressure as the rear suspension.

If the main system pressure fails the anti-sink valves will close and now the rear brake accumulator sphere will supply the brake doseur valve and allow the rear brakes to still work.

So the purpose of the sphere is to re-instate the rear brake redundancy that was already there in the system before anti-sink was introduced. A corollary to this is that if the rear brake accumulator sphere is stuffed, should system pressure fail you will lose both front and rear brakes together...

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Re: Xantia MK II - Low rear brakes efficiency

Post by lancia58 » 09 Jul 2019, 13:54

So if I accelerate then engage to Neutral and switch off the engine only the rear brakes will brake ( given the rear accumulator is OK ) ? If this is correct can it be used to test the rear brakes ?

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Re: Xantia MK II - Low rear brakes efficiency

Post by Mandrake » 09 Jul 2019, 14:00

lancia58 wrote:
09 Jul 2019, 13:54
So if I accelerate then engage to Neutral and switch off the engine only the rear brakes will brake ( given the rear accumulator is OK ) ? If this is correct can it be used to test the rear brakes ?

No.

If you switch off the engine (don't do that by the way as you'll lose power steering) the front accumulator sphere will provide enough stored pressure for many activations of the brakes.

I'm talking about catastrophic loss of system pressure due to for example a burst pipe. A broken accessory belt that stops the pump running would still leave you with the stored pressure in the front accumulator sphere to stop safely.