Xantia rear suspension

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Mandrake
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Post by Mandrake »

I don't think the anti-sink valve has closer tolerances than a height corrector - I think the reason its less "leaky" is that it has an o-ring seal on the shaft, while the height corrector is seal-less and relies entirely on micron machining of the shaft and bore.

(Presumably seals are omitted to make it more sensitive and precise in response than it would be if it had o-rings - also to make it last a long time with no degradable component that is going to prevent it working when it wears out...)

The anti-sink sphere has a number of effects but its clear that its primary purpose is that of a rear brake accumulator, and that the other effects it has are secondary and/or side effects.

When the main system pressure is lower than suspension pressure the anti-sink valve closes and isolates the rear height corrector, rear suspension, and brake doseur valve from each other. (3 way isolation - all are isolated from each other)

As both the height corrector and doseur valves are sources of significant leakage (especially the latter) this stops the suspension sinking.

Unfortunately the rear suspension is now no longer connected to the brake doseur so there is no source of supply for the rear brakes, and this is a safety issue.

In an older Citroen if your main system pressure suddenly failed you would lose front brakes, but rear brakes would be supplied from the rear suspension, and be enough to stop the car. With an anti-sink valve and no anti-sink sphere you would lose rear brakes as well.

The anti-sink sphere is connected to the brake doseur valve permanently even when the anti-sink valve is closed and acts as a rear brake accumulator.

Unfortunately it does increase the time to lift in the mornings as the doseur valve bleeds all the stored pressure from the anti-sink sphere overnight and that has to be replaced to lift the suspension. (As I noticed when I replaced my punctured anti-sink sphere and it started taking longer to lift in the mornings...)

The slight drop that you notice when the anti-sink valve opens is suspension pressure rushing into the anti-sink sphere to equalize the pressure. If the sphere is very low in gas but not quite punctured, it will drop a lot more than if the sphere is gassed properly.

Regards,
Simon
Simon

1997 Xantia S1 3.0 V6 Auto Exclusive in Silex Grey
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Richard Stevens
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Post by Richard Stevens »

Jeremy:
Thanks for the anti-sink sphere explanation. So the anti-sink valve is a pressure maintaining valve, in the parlance I'm used to (a glorified non return valve).
The system just gets more interesting......
Strangely on replacing the two main rear spheres I found that the fluid level dropped considerably in the reservoir, though I lost very little.
I wonder if I just got rid of a lot of air (or nitrogen) that was in the system. Anyway now the fluid level is correct - I had overfilled it when I replaced the fluid some time back.

Regards
Richard
Richard Stevens
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Post by Richard Stevens »

I thought I'd posted a reply a couple of days ago, but it's not there, so....
Thanks for the info, that's perfectly plain, another piece of the jigsaw!

I must say the suspension is much better since I changed the main rear spheres, now I'm finding the front a little firm.....

Richard
'99 Xantia HDI Exclusive
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