Citroen XM Forum

Gabor, that was always my reading of the descriptions - yours and Citroen's.

As you say, when engine is running (or car being driven) the a/s sphere has no function. It is there to provide a positive pressure differential for the a/s valve to close when the system pressure eventually reduces. In this sense, it backs up and rams home the effect of the springs in the a/s valves, holding them positively shut.

It must also be true, I guess, that that the rear brakes don't need a back-up/pressure reserve when the car is parked, or the engine is switched off. There's more than adequate braking available at the front, when parked, and certainly when on-the-move. (Rear brakes bring little to the party most of the time, anyway.)

The functioning of the SC/MAC system would definitely make a splendid Advanced Level question at the Citroen Academy ("The SC/MAC additional sphere makes a much-needed contribution to rear brake performance. Discuss.").

A good idea for a supplementary Section on the late-Xantia valves. No shortage of text and drawings as source material!

djg wrote:It seems that *all* statements are essentially valid and any contradiction is probably just apparent. The misunderstanding might come from the expression of "feeding the rear brakes". The sphere does that under some circumstances but not when the car is in motion and the brakes are potentially operating.

To be pedantic, the anti sink sphere is connected to the brake valve 100% of the time, regardless of whether the anti sink valve is open or closed.

djg wrote: When the car is stopped, the valve still stays open for some time. It only closes when the main pressure decreases enough to drop below the suspension side pressure. The anti-sink sphere simply provides additional pressure to make sure that the valve stays closed and doesn't reopen. It does so by adding an extra reservoir to the brake circuit side of the valve (because the brake valve, being usually the most leaky element, would deplete it). So, in this sense, it does feed the brake circuit, yes, but not in the sense that it would influence braking because the car had been stationary for long by then.

The sphere, by itself, when functioning normally, has no influence on the sinking, that's true. But if it doesn't hold pressure, the anti-sink valves will reopen and the car will sink rapidly. So, a bad sphere does have influence on sinking, and quite a drastic one at that.

That is not quite correct, The anti sink valve has 3 chambers internally (line pressure, brake valve pressure and rear suspension pressure), which when closed are all isolated from each other. the anti-sink valve is held closed by a combination of spring pressure and the pressure in the rear suspension. The pressure in the braking circuit (anti sink sphere) does not significantly influence the opening or closing of the valve. This can be demonstrated by the fact that a car with a completely flat anti-sink sphere does not sink.

djg wrote: Now, the question is, how is this different with the later Xantia valve? I could modify the illustrations in the description to account for that.

It was the Xantia hydractive valve that was modified, not the anti-sink valve. See: https://frenchcarforum.co.uk/forum/view ... 81#p494381" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false; for a full description.

Thanks for the info, I don't normally do anti-sink.
It does make sense as if the supply pressure suddenly failed, the anti-sink valve would close and leave the rear brakes without pressure to operate as the rear spheres would be isolated.
From my understanding, the anti-sink sphere also ensures the valve remains closed by ensuring the pressure differential across the shuttle. If the sphere looses pressure, the valve can open once the rear system pressure is depleted below that of the feed circuit.

rigidity sphere at rear replaced along with both outer rears that were flat and brakes bled and this week brand new acc sphere replaced problem still there.

car appears to sink as soon as the engine is started then slowly rises sometimes putting in intermediate brings it up quicker and sometimes brings the low pressure light on.


the rear height corrector is brand new as is hp pump.

After all the discussion about SC/MAC (anti-sink) - sorry, this kind of thing happens a good bit here! - was the SC/MAC sphere ever replaced?

If there's still some gas pressure in it (and this sphere does tend to have quite a long life), it will probably be managing to keep the anti-sink valve closed when the engine is switched off, and the main system pressure falls away. To that extent, it will work as intended. However, when the engine is started, the main system pressure won't have to climb very high before it exceeds the SC/MAC sphere's (reduced) pressure, and when it does, the car could sink as the valve opens. As the system pressure rises, so will the car. It will do this every time the car is started after a significant period of park-up.

So if the SC/MAC sphere hasn't been renewed, I think a good time to do it.
It's located in the rear LH corner of the car, next to the LH side of the subframe. Access isn't great, but do-able if the spare wheel and carrier is removed.
Several write-ups on the site about removing the sphere. One here:
viewtopic.php?f=7&t=7165#p83110.

The sphere is different to the other spheres on the car, with a 3.5mm pipe union screwing straight into it, and a large nut securing it to a supporting bracket. Be careful not to twist the pipe when removing it. Use WD40 etc first.
The SC/MAC ("anti-sink") sphere is available from most of the usual sources. It's 400cc/50bar/no damping, direct pipe entry.
Bleed the back brakes after fitting.

found it what a stupid location!!

the hp pipe to it is new but looks like the sphere or bracket have never moved!

i am wondering if it is possible to just remove the hp pipe as its quite new
undo the big nut and then manipulate the sphere out without removing the bracket?

next qusetion is where to source a replacement?

i will look on ebay but not sure if i will find one.

Citroen pt.no. is 5293 04, and still available. Likely expensive.
AEP in Glagow (who do IFHS spheres) don't seem to list it.

Several to choose from on Mister Auto UK: https://www.mister-auto.co.uk/suspensio ... 11136.html
I'd go for Bolk, BOL-E-111136 (Mister Auto own brand; MA owned by PSA) or Febi-Bilstein, 22503 (not sure who makes theirs).
Lizarte haven't been everyone's favourite, and ditto Monroe (where it's unclear on the shape of the item, saucer or classic).

I removed the bracket, and drifted the big nut loose with a small cold-chisel.
Some owners have dropped that side of the rear subframe to give better access.
You are right; a sh*tty location.
I replaced just 2 of the 3 bracket bolts, to make it easier to remove 'next time'.