Rear anti-sink?

Cpt_Spalding · Post by **Cpt_Spalding** » 05 Oct 2007, 09:44

Ok,

I had the rear anti-sink sphere replaced at a citroen specialist last week. Gave it to them coz I wouldn't have the time or the confidence to replace the pipework if I cracked the connecting pipe while doing the work.

The work was done, but I'm still having the same issue.

When I start my car for the 1st time each day the rear end sinks & takes about 20 seconds to pump back up once it has dropped.

I thought I read on a previous thread that the rear anti-sink sphere was the issue here & that if it carried on like this, although it shouldn't be an issue in itself, it would cause other components to be overworked & possibly fail.

Have I just thrown £60 away? What is the real problem (apart from my thickness)?

Thanks.

nick · Post by **nick** » 05 Oct 2007, 09:53

I made this mistake too when I had my Xantia. The rear anti-sink sphere is apparently misnamed, as it exists to provide a reserve of pressure to the rear brakes, nothing to do with the actual suspension anti-sink function.
The symptoms you're experiencing are usually caused by the accumalator sphere being flat, rather than the anti-sink.

Cpt_Spalding · Post by **Cpt_Spalding** » 03 Jan 2008, 09:30

I've now had both the anti-sink and the accumulator spheres replaced & I'm having the same symptoms.

Basically, if the car hasn't been started for a day or so, the rear end sinks immediately on starting & pumps back up after about 20 seconds.

Does anyone have any (further) suggestions on how to rectify this? I'm assuming this isn't normal.

Thanks in advance.

sdabel · Post by **sdabel** » 03 Jan 2008, 11:26

My Xm does this, and it has no rear anti sink sphere so it is not that! Also I have replaced the main accumulator and that made no difference. May at least help you rule some parts out.

regards
sean from very hot Adelaide

Post by **aerodynamica** » 03 Jan 2008, 15:30

I used to see a guy start a Xantia up each morning that did this. The car was a 'N' reg ('96) Diesel and it would stay up all night from when it was parked. It would be up in the morning and he'd get in it would sink a little normally and he'd start it - count to 2 and the rear would slowly lower right down to low.

It would then rise up to its normal height but sometimes he'd drive off with it still slammed.

It always struck me that it was caused by air trapped somewhere in the rear circuit(s) it made me think there was some hydraulic valve that opened shortly after switch on that connected the rear suspension pressure to a previously isolated circuit that had some trapped air in it and as soon as the higher suspension LHM circuit met it, the trapped air compressed and the rear hit the floor. Bit like air in the rear brakes - it's isolated from the rear suspension until the brake is pressed then the susp pressure joins to the brake circuit including its bubble of air and the rear goes down as the air compresses.

If the anti sink valve is a thing that is shut until the accumulator and line pressure are high enough to open it and reconnect the circuit then any air trapped in that bit of the circuit would behave like this when the pressure came up and opened the valve. If the anti sink sphere was leaking it could bubble off gas into the circuit to be opened when the anti sink valve did. With a new anti sink sphere fitted how would we be getting any trapped air out? maybe it needs bled.

If an XM without anti sink does it, again perhaps it's some valve that only opens when the ignition os turned on and then trapped air meets suspension pressure. Maybe the anti jolt valve?

Anyone see what I'm on about? I know it's a long response..

jeremy · Post by **jeremy** » 03 Jan 2008, 16:37

You need a good textbook on hydraulic Citroens.

http://www.tramontana.co.hu/citroen/

Follow link to technical guide - top LH corner.

nick · Post by **nick** » 04 Jan 2008, 00:20

My Xantia did this when I first bought it in 2000. Not long after acquiring it I changed the anti-sink, accumulator and rear suspension spheres, and also changed the fluid and cleaned the filter in the LHM tank. It seemed much happier after that, and the problem never reappeared in the following 6 years I owned the car, despite the spheres subsequently losing pressure and being replaced yet again just before I sold it.

I had assumed it was changing the accumulator sphere that made the difference, but it makes me wonder now if that was really the case . .

Post by **CitroJim** » 06 Jan 2008, 13:09

This is how I understand it. I may be barking up the wrong tree though...

The anti-sink (A-S) sphere is indeed there to give the rear brakes a reserve of pressure but it does something else besides. It ensures the rear anti-sink valve remains closed until good main hydraulic pressure has built. The rear suspension hydraulic circuit also feeds the rear brakes via the brake dosuer and the dosuer is normally the most leaky part of the entire system. If the A-S sphere were not there, the doseur would exhaust all residual pressure on the suspension sphere side of the anti-sink valve and the valve would open and the rear sink. The A-S sphere keeps the rear anti-sink valve firmly closed and maintains a residual pressure in the rear brake circuit. It does not need much residual pressure to keep the valve closed. When you start up, a burst of high pressure is deliverd to the rear height corrector and A-S valve and it flies open (due to a large pressure differential) and in doing so, the stored pressure in the rear suspension spheres is rapidly exhaused, trying to equalise the (now lower) pressure in the A-S sphere and rear brake circuit ahead of the main accumulator sphere getting there to do the job.

There are four possibilities for sinking on startup, in addition to the main accumulator being weak or the A-S sphere itself being flat. These are:

A badly leaky brake dosuer valve;
Minor hydraulic leaks anywhere between the rear A-S valve, the A-S Sphere, dosuer and rear calipers;
Flat rear suspension spheres; (nick, why your problem cleared perhaps?)
An "iffy" security valve.

An LHM change preceded by a good flush with hydraflush is also highly recommended as is a very careful study of the link supplied by Jeremy

As I say, I may not be 100% right in how it all works... Mandrake, your thoughts as resident hydraulic guru

Cpt_Spalding · Post by **Cpt_Spalding** » 06 Jan 2008, 15:13

Wow!

Thanks to all so far. Not sure what to do next but it won't be for a little while yet. Xmas & hefty insurance bill (too many points & a recent prang) have seen to that.

In addition, if she's been started & run already that day, the sink on start-up doesn't happen. Only if it's been left for a day.

Stempy · Post by **Stempy** » 06 Jan 2008, 23:32

I think the anti sink sphere is badly named and should really be called the rear accumulator.

I would say your problem is most likely to be duff rear suspension spheres. I've had this happen twice and both times new rear spheres cured the problem.

steelcityuk · Post by **steelcityuk** » 07 Jan 2008, 11:10

I would have said that starting the engine starts to re-pressurizers the system and therefor open the anti sink valve which I suppose would allow the pressure that was isolated by the anti sink circuit to dissipate and so the rear end sinks. 20 seconds isn't bad after all night.

If you drop the Xantia down to the bottom for 2 minutes then select normal how long does that take? Don't forget a 6+2 pump doesn't have much capacity compared with the older Citroens fitted with the FDV.

Steve.

Cpt_Spalding · Post by **Cpt_Spalding** » 08 Jan 2008, 22:02

Stempy wrote:I would say your problem is most likely to be duff rear suspension spheres. I've had this happen twice and both times new rear spheres cured the problem.

You may well be on the button there. I had a look through the receipts yesterday (I inherited a file of receipts that goes back to dot for the car) & there's no record of the rear spheres ever being changed & it's done 125K now.

Post by **CitroJim** » 14 Jan 2008, 13:47

A little post-script to this thread.

My 2.1TD very recently began sinking very slightly on start-up. It was only because I parked it for two days on my concrete drive (whilst I gave my Activa a few outings) and noted a very small spot of LHM on the concrete (no bigger than a 10p piece). Investigation found a very small weep from the accumulator sphere and an equally tiny weep from one of the pipe unions on the pressure regulator.

I replaced the sphere seal and nipped up the weepy union and touch-wood, the problem is now resolved.

citronut · Post by **citronut** » 24 Jan 2008, 22:36

dose any one have diagrams (scematis) possibly with direction of flow for xantia hydraulic circuits
regards malcolm

falling-out-with-my-car · Post by **falling-out-with-my-car** » 09 Apr 2008, 20:09

Does anyone think this all might be the rear end compensating for the installation of a heavy driver?

Regards Nigel.

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