Hello all.

Have my car back from the menders (and my trousers back from the dry cleaners) after the high pressure petrol leak. I've a couple of questions

The first is about the rear and braking. Now, to my mind the car behaves the 'wrong way round' when braking, in that the rear sits down when you brake and raises up again when you move off. Sounds like Citroen kind of odd, but is it right? In addition, when stuck in start/stop traffic the rear can drop suddenly when the car isn't moving. And when I say drop, I don't mean that it sinks gradually, I mean it goes down in a second. It doesn't bottom out, rather it feels as though the fluid loses interest in keeping the back up and rushes off elsewhere. No fluid loss, no other symptoms, car drives perfectly after it's happened, though my brain does tell me the car feels slightly lower behind me. Thoughts welcome.

Second, on some mornings - and I'm yet to work out what causes it - the suspension sounds 'clattery'. The ride seems perfectly good, but any up/down travel is accompanied by a rattling noises. I could be imagining it, but kicking the car into 'sport' mode makes it worse. Driving the car for a little distance tends to cure it, and then everything is beautifully quiet.

Hard to describe complex Citroen issues in words but I hope I managed it!

Regards,
Phil.

No Phil, good description! I think I understand what you mean about the dropping rear - I've had this too, albeit on the CX but the cause might be the same:

I was getting normal braking first thing in the morning but after some miles and once everything had heated up, in traffic stop and start, successive brakes would cause the back to lower: It was only in the last foot of forward movement before stop. It got worse and worse and became really annoying. I thought it was the brake doseur valve as I checked its leak back and found it to be high. Fitted a brand new one and found no improvement (except the rear suspension stays up overnight) on the road it was the same.

I'd had a BX with a more severe form of this in the past: the rear end would hit the deck at the first suggestion of braking - I bled the rear brakes and it cured it after rather a lot of air came out (I'd replaced the rear spheres previously) But the CX didn't change after a very long bleed!

Still hitting the deck in traffic I was a bit stuck. I'd noticed the braking effort was not up to scratch overall and thought maybe the brake accumulator - changed, bled - no difference, still lowering at the rear.

Then I engaged the brain. I thought the use of trailing arms on all Citroens with the brake caliper along the arm would tend to cause a lowering effect anyway but generally doesn't - why was that? - it's because the rear wheels move away from the fronts where the rear arms go up and so the front needs to 'roll away' from the braked rears to allow the braking effect of the rear to cause the arms to pivot down. The stronger, earlier front brakes engaged always prevent this so why wasn't mine? Maybe the front brakes were not up to scratch! - I hadn't even thought to look.

I checked the fronts and lo and behold they were oil contaminated! it explained the bad braking effect but explained why the rear sits down when I braked! the rear brakes were working harder than the fronts!

All I can suggest is first bleed the rear brakes and second check the front brakes!

Only other thing is, do you sit in traffic with the foot brake on? if so it can cause a phenomenon where the rear suspension suddenly falls down or shoots up when you take your foot off. It's more likely on an automatic. It's caused by the rear calipers grabbing the disks and holding them tight while the rear suspension is a little under or over correct height. The height corrector opens and starts taking LHM from the rear suspension but it will not lower with the calipers holding it so the corrector stays open and too much LHM is removed. As soon as the brake comes off it falls to the stops. The opposite happens if it tries to raise the rear a little with the brakes on - corrector keeps adding LHM to raise but brakes holding it - release brake and it jumps up high. You can demonstrate it too (o the excitement!) by putting the height up from normal to high and holding the foot brake. The front will go up as expected but the rear will stay. That is until you release the brake where it'll shoot up very quickly and top out. Holding the brake after selecting low does the opposite - it will not go down until you release the brake, however, it will eventually lose rear brake pressure too since it's sourced from the rear suspension - as the brakes lose pressure you'll hear a groaning noise as the brake pads start to slip.

It happens on autos in traffic because of the creep sometimes pushes the rear a tiny bit down and then inevitably you stop on the brake and hold (E.g. turning right across the oncoming traffic waiting for a gap).

Great, thank you Graeme. Seems like you explained away my first question in one hit. You are correct that I sit in stop/start traffic with my foot on the brake and that it's an auto

Hi Y'all,

New to this Forum.

We have been concerned about these self same very disconcerting symptoms with our M reg Diesel XM, VSX auto estate. Your comments from Aerodynamica appear to suggest that this is normal behaviour?

It has been looked at by a local Citroen specialist who finds nothing wrong?

However, we've owned this one for about 5 years but have only become aware of this phenomenon over the last couple of years or so. We also owned a G reg 2Ltr Petrol hatchback for a number of years, which never appeared to have this problem?

Not arguing, just seeking reassurance I guess.

Keep safe and be lucky.

Gerry.

Hi Gerry, welcome to THE forum!

I think aerodynamica explained it very well. Whenever you put some outside influence on the hydraulic system - like deliberately holding it in a higher or lower than normal setting, it will behave unpredictably when that outside influence is removed. Come to think of it, it's entirely predictable, but it seems strange at the time.

Because the rear brakes do so little work on the XM, it is not unusual for them to seize up. When they aren't working they cannot influence the behaviour of the rear suspension. Hope that gives you something to think about.

Derek

Hi Gerry

And welcome aboard. Put a post in the wanted section someone will have a pair of handles for you im sure.

Regards
D

Hi Guys.

Have had the rear end height fluctuation in traffic on several of mine. Present one has a new (to me) symptom -- there is a distinct delay in the rear brakes coming into play. Most noticeable at speed, if you press the pedal, the front dips, then a second or so later you can feel the back go down.

The other quirk I've noticed on several of them is if you move off with the car set to high, then change it to normal as you pull away, the brakes seem to disappear for an instant, as though the sytem is confused -- could be nasty in the wrong place.

Mike.

Hi Mike!
You could try to bleed the rear brakes.
I had the same sympton after changing LHM this autumn.
After bleeding evrything went back to normal.

/Erik J

The rear brakes take their pressure from the suspension cylinder side of the height controller. So if you start off on high and then select normal, the height controller opens the suspension and the brake line to return to reservoir. Therefore no brake pressure is available until the HC returns to its neutral holding position.

Derek

Hi Mike,

Sorry I don't have an answer to your delayed braking, but Erik's bleeding suggestion (if you'll excuse the espression ) may provide a cure.

Derek

Ive always found the brakes on the Xm to be excellent, infact possibly one of the best ive experienced. Even when you stamp on them there is no dipping, it just pulls up very level, without any fuss and very quickly. Which must mean both front and bake brakes work on a similar pressure?

Usually when braking hard the front of a car dips dramatically upsetting the stability of the car, but the Xm is very level and controlled.

Its interesting to watch peoples reactions in nearby cars when the car suddenly drops or raises height when pulling away from junctions.

Do all Xms (not sinking XMs) groan sometimes when you leave them? Sometimes when i park up and wait in the car, or even after i get out, it groans for a few seconds. I thought its something to do with the anti-sink, it really gets some funny looks as people walk by

Hi Citroenmad,

"Which must mean both front and bake brakes work on a similar pressure?"

No, as approximately 65% of the car's weight is carried on the front wheels, equal brake pressure would cause the rear wheels to lock (ignoring the effect of ABS) under quite moderate braking.

The front brakes take their pressure from the main accumulator, which also supplies emergency pressure in the event of hydraulic failure. The rear brakes take their pressure from the rear suspension which is Citroen's clever way of making them load sensitive.

The anti dive property is simply the torque of the rear brakes acting on the moment arm of the rear suspension arms to hold the back end down.

My cars hiss quietly as they correct for passenger/driver weight being removed. If yours are groaning it may be your rear arm pivot bearings complaining. This was a built in fault, no way to lubricate them. Some owners have fitted grease nipples.

Derek