Righto, it seems after a bit of searching, there is no definitive answer to what the 'anti sink' sphere does at the rear of the car.
From what I've gathered, it functions as an accumulator sphere for the rear brakes, but could also be responsible for the rear sinking when the car is started if it's flat. Is this right?
So, what then controls the anti sink valve? How is this opened by the starting of the engine to allow the rear to drop and rise again?
And that leads me on to another question...if the anti sink valve closes when the engine is off, how can the rear height corrector operate from accumulator pressure if the rear of the car is isolated?
Just a few queries really!
Chris.
'Anti sink' spheres...
Moderator: RichardW
I think its basically just a sales gimmick!! Apparently market research indicated that people didn't like their cars sinking and wanted them to look normal when they came back to them, so it was added part way through the Xantias production. It made the Xantia more mass-marketable to non Citroen type people in that it didn't do anything weird in a Citroenly sort of way.
I believe you can run perfectly well with the anti sink system blanked off, I'm sure I saw something on here about how to do it.
I believe you can run perfectly well with the anti sink system blanked off, I'm sure I saw something on here about how to do it.
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Kowalski
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The anti-sink sphere was added when anti-sink was added to the car, hence the anti-sink name.
It is as you've said a rear brake accumulator sphere and little more.
The anti-sink valves don't operate when the engine goes off, they operate when there isn't enough pressure in the accumulator sphere any more, this is why with the engine off you can test the accumulator sphere by sitting in the boot and seeing if it rises or not.
My theory about flat accumulators making the car sink goes as follows. When you start the engine the pump pressurises the accumulator sphere. If the sphere is low on gas it won't store a lot of pressure, i.e. its effective volume is reduced and it can't deliver the full amount of LHM at the required pressure. Once the accumulator is pressurised the anti sink valves open, and the height correctors can request pressure to get the car up to height. If the accumulator sphere is flat, it cant deliver the volume that the rear suspension needs and you end up with the LHM going the wrong way, i.e. out of the rear suspension instead of into it, where it goes I can't say for sure but it possibly goes into the front suspension or the brake circuit.
It is as you've said a rear brake accumulator sphere and little more.
The anti-sink valves don't operate when the engine goes off, they operate when there isn't enough pressure in the accumulator sphere any more, this is why with the engine off you can test the accumulator sphere by sitting in the boot and seeing if it rises or not.
My theory about flat accumulators making the car sink goes as follows. When you start the engine the pump pressurises the accumulator sphere. If the sphere is low on gas it won't store a lot of pressure, i.e. its effective volume is reduced and it can't deliver the full amount of LHM at the required pressure. Once the accumulator is pressurised the anti sink valves open, and the height correctors can request pressure to get the car up to height. If the accumulator sphere is flat, it cant deliver the volume that the rear suspension needs and you end up with the LHM going the wrong way, i.e. out of the rear suspension instead of into it, where it goes I can't say for sure but it possibly goes into the front suspension or the brake circuit.
Ah, well that's interesting because I have just changed my acc sphere last night to a new one and the ticking is now at 2 minute 20 sec intervals. The rear still sinks the second the engine starts with kind of a clunk noise from underneath which I'm assuming is the anti sink valve opening and the LHM rushing somewhere! Someone else mentioned that a flat 'anti sink' sphere could also rob the system of LHM pressure when te valve opens, like what you detailed above. Do you reckon that is the case?
This is taken from another website...The LHM returns to the reservoir via the brake distributor valve. If that cannot keep the pressure in the rear suspension but lets the fluid return, the rear end will sink fast. This is not a problem in itself: if the car sits up fast when started, has no other suspension or braking problems, there is no need to rush to the workshop.There are quite a few other factors influencing the sinking as well. Flat rear spheres, old LHM and hot weather also make it sink faster.
The question of flat rear supspension spheres causing sinking always amuses me. Just think about it - you stop your car and leave it in the normal position - spheres are compressed to support the load - why should their pressure make any difference to how fast it sinks. Even if they are flat exactly the same will happen. What can make a difference and frequently does is wear in the height correctors.
Jeremy
Jeremy
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FrenchLeave
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Quote: " I think its basically just a sales gimmick!!"
I think not. Anti sink was introduced with the introduction of the split hydraulic pump. With only two cylinders of the pump available for the suspension it takes for ever to raise it from fully collapsed to normal height. The anti sink valves isolate the suspension cylinders and hydractive valve block from the height correctors as soon as the pressure from the accumulator falls below that in the suspension system.
With the rear brakes operated by pressure from the rear suspension, this posed a problem in the event of pump failure whilst moving because the anti sink valves stopped that pressure being available for the brakes, hence the need to have an alternative pressure supply - the anti sink sphere which is piped in between the rear height corrector and the anti sink valve.
Looking at the rear system only, when the engine is started, pump output replenishes the accumulator. As soon as the pressure from the accumulator (via the security valve) exceeds the pressure trapped in the suspension, the anti sink valve will open, connecting the suspension to the height corrector and to the anti sink sphere. My hypothesis is that if the A/S sphere is exhausted, the accumulator should replenish it; but if the accumulator is flat, the pump has to do the job and with only two cylinders that will take a few seconds. In that interval the pressure fluid in the suspension flows out to the accumulator through the open A/S valve and the suspension drops.
Feel free to shoot me down.
Regards, Derek.
I think not. Anti sink was introduced with the introduction of the split hydraulic pump. With only two cylinders of the pump available for the suspension it takes for ever to raise it from fully collapsed to normal height. The anti sink valves isolate the suspension cylinders and hydractive valve block from the height correctors as soon as the pressure from the accumulator falls below that in the suspension system.
With the rear brakes operated by pressure from the rear suspension, this posed a problem in the event of pump failure whilst moving because the anti sink valves stopped that pressure being available for the brakes, hence the need to have an alternative pressure supply - the anti sink sphere which is piped in between the rear height corrector and the anti sink valve.
Looking at the rear system only, when the engine is started, pump output replenishes the accumulator. As soon as the pressure from the accumulator (via the security valve) exceeds the pressure trapped in the suspension, the anti sink valve will open, connecting the suspension to the height corrector and to the anti sink sphere. My hypothesis is that if the A/S sphere is exhausted, the accumulator should replenish it; but if the accumulator is flat, the pump has to do the job and with only two cylinders that will take a few seconds. In that interval the pressure fluid in the suspension flows out to the accumulator through the open A/S valve and the suspension drops.
Feel free to shoot me down.
Regards, Derek.
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by jeremy</i>
The question of flat rear supspension spheres causing sinking always amuses me. Just think about it - you stop your car and leave it in the normal position - spheres are compressed to support the load - why should their pressure make any difference to how fast it sinks. Even if they are flat exactly the same will happen. What can make a difference and frequently does is wear in the height correctors.
Jeremy
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Not the rear spheres mate, the anti sink one. I know that tese are holding the weight of the car. They do hold it fine, just as soon as you tuen the key and start the car, the rear sinks.....a common problem.
My hypothesis was pretty much as Frenchleave put it although I'm not sure if it's 100% correct. When you start the car, the pump primes the acc sphere first then the system is at operating pressure. Whether the acc sphere is flat or not, surely that means the acc sphere is primed before the anti sink valve is opened and as such shouldn't rob any fluid pressure from the rear suspension causing it to sink.
Perhaps it is a requirement for BOTH sphere's (anti sink and acc) to be fully charged and healthy for the system not to sink. LHM pretty much decides on its own which end to flow to first, and as such, if there is a flat sphere somewhere (not a corner axle one but one of the other 2...anti sink or acc) then the pressure will suddenly be lost to that sphere as soon as it is added to the system when a valve opens, causing a momentary loss of pressure as that sphere fills up and the arse end to sink. The flatter the sphere, the more LHM it will gobble and the more it will sink.
So basicvally what I'm saying is, perhaps to stop the rear from sinking when the car is started, both sphere's must be in good condition, the accumulator and the anti sink, otherwise LHM pressure will be robbed by one of them.
How's that sound?
The question of flat rear supspension spheres causing sinking always amuses me. Just think about it - you stop your car and leave it in the normal position - spheres are compressed to support the load - why should their pressure make any difference to how fast it sinks. Even if they are flat exactly the same will happen. What can make a difference and frequently does is wear in the height correctors.
Jeremy
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Not the rear spheres mate, the anti sink one. I know that tese are holding the weight of the car. They do hold it fine, just as soon as you tuen the key and start the car, the rear sinks.....a common problem.
My hypothesis was pretty much as Frenchleave put it although I'm not sure if it's 100% correct. When you start the car, the pump primes the acc sphere first then the system is at operating pressure. Whether the acc sphere is flat or not, surely that means the acc sphere is primed before the anti sink valve is opened and as such shouldn't rob any fluid pressure from the rear suspension causing it to sink.
Perhaps it is a requirement for BOTH sphere's (anti sink and acc) to be fully charged and healthy for the system not to sink. LHM pretty much decides on its own which end to flow to first, and as such, if there is a flat sphere somewhere (not a corner axle one but one of the other 2...anti sink or acc) then the pressure will suddenly be lost to that sphere as soon as it is added to the system when a valve opens, causing a momentary loss of pressure as that sphere fills up and the arse end to sink. The flatter the sphere, the more LHM it will gobble and the more it will sink.
So basicvally what I'm saying is, perhaps to stop the rear from sinking when the car is started, both sphere's must be in good condition, the accumulator and the anti sink, otherwise LHM pressure will be robbed by one of them.
How's that sound?
My 2pence of ...
If a sphere is flat it wont take many mL to fill it. Mind you that the term "flat" sphere means there are no gas pressure left to press out any LHM as soon as the system is at rest. Meaning only a few mL will be pressed out at rest.
Only the suspension pressure *could* press out the LHM - but not. Because it is the LHM contents in the cylinders that are pressed out while the car slowly sinks.
In the longterm process of a sphere going flat it will slowly take up more & more LHM to compensate the lost volume. This is seen as the occasional need for LHM topping up with no other appearent reason.
If the flat (or near flat) spheres then had to press out all the LHM during system rest - you would truly have a very messy LHM reservoir overrun every time you stopped the engine.
But this does not happen ...
If a sphere is flat it wont take many mL to fill it. Mind you that the term "flat" sphere means there are no gas pressure left to press out any LHM as soon as the system is at rest. Meaning only a few mL will be pressed out at rest.
Only the suspension pressure *could* press out the LHM - but not. Because it is the LHM contents in the cylinders that are pressed out while the car slowly sinks.
In the longterm process of a sphere going flat it will slowly take up more & more LHM to compensate the lost volume. This is seen as the occasional need for LHM topping up with no other appearent reason.
If the flat (or near flat) spheres then had to press out all the LHM during system rest - you would truly have a very messy LHM reservoir overrun every time you stopped the engine.
But this does not happen ...
So when the engine is stopped, the lack of air pressure in the duff spheres means no LHM is forced out of them. So, when the engine is started, the pump merely has to slightly top up the LHM in them (perhaps) and so, theoretically, no LHM should 'rush' anywhere when the anti sink valve opens, other than perhaps back to the reservior.
So, how does this explain the wierd sinking when the engine starts? Where does the LHM go to allow the rear pressure to drop and the ass to sink, before the pressure can rise again and pump it back up? Does it return to the tank un-hindered, before the pump re-primes the acc sphere and returns the rear pressure to normal? Or is there some thing in the way that's meant to stop the LHM simply racing back to the tank when the anti sink valve is open.....
So, how does this explain the wierd sinking when the engine starts? Where does the LHM go to allow the rear pressure to drop and the ass to sink, before the pressure can rise again and pump it back up? Does it return to the tank un-hindered, before the pump re-primes the acc sphere and returns the rear pressure to normal? Or is there some thing in the way that's meant to stop the LHM simply racing back to the tank when the anti sink valve is open.....
I found this document (Swedish unfortunately) describing the SC-MAC ('anti-sink') system:
http://www.citrotech.se/data/pdf/xantia ... sc-mac.pdf
Fig. 3.1.2 shows the anti-sink valve (4), the height corrector (5), connection to brake valve and SC-MAC ('anti-sink') sphere (a).
As I understand it, the anti-sink valve disconnects the suspension spheres/struts from the rest of the system and disconnects the accumulator side from the SC-MAC('anti-sink') side when the pressure from the accumulator side is lower (or not somewhat higher) than the pressure from the SC-MAC ('anti-sink') sphere.
So when the car has ben parked for a while the pressure maintained by the SC-MAC sphere is low due to leakage through the brake valve.
When the car is started, the pressure builds up in the accumulator and when the accumulator pressure is above the SC-MAC pressure (which is below the suspension pressure) the anti-sink valve reconnects the suspension to the rest of the system and LHM flows from the suspension to the SC-MAC sphere and the rest of the system due to the pressure difference.
I can't check if this is the case because my Xantia doesn't have all that silly anti-sink stuff
Søren
http://www.citrotech.se/data/pdf/xantia ... sc-mac.pdf
Fig. 3.1.2 shows the anti-sink valve (4), the height corrector (5), connection to brake valve and SC-MAC ('anti-sink') sphere (a).
As I understand it, the anti-sink valve disconnects the suspension spheres/struts from the rest of the system and disconnects the accumulator side from the SC-MAC('anti-sink') side when the pressure from the accumulator side is lower (or not somewhat higher) than the pressure from the SC-MAC ('anti-sink') sphere.
So when the car has ben parked for a while the pressure maintained by the SC-MAC sphere is low due to leakage through the brake valve.
When the car is started, the pressure builds up in the accumulator and when the accumulator pressure is above the SC-MAC pressure (which is below the suspension pressure) the anti-sink valve reconnects the suspension to the rest of the system and LHM flows from the suspension to the SC-MAC sphere and the rest of the system due to the pressure difference.
I can't check if this is the case because my Xantia doesn't have all that silly anti-sink stuff
Søren
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Chrispy</i>
So when the engine is stopped, the lack of air pressure in the duff spheres means no LHM is forced out of them. So, when the engine is started, the pump merely has to slightly top up the LHM in them (perhaps) and so, theoretically, no LHM should 'rush' anywhere when the anti sink valve opens, other than perhaps back to the reservior.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
There is an important distinction between a sphere which has *NO* gas pressure left at all (usually due to the flexible diaphram rupturing and letting oil into the "top" section of the sphere as well, and letting the remaining gas escape away through the hydraulic system to eventually bubble out of the resoviour) and a sphere that is just very low on gas but is still otherwise functioning.
Using the accumulator sphere as an example, if there is NO gas whatsoever, then the sphere is full of oil when the system is at normal operating pressure, and when the motor is off and the pressure is slowly leaking away, the pressure drops *quickly*, (since there is no compliant medium to maintain the pressure as the oil is used to maintain system leakage) but almost no oil will come out of the sphere, as there is no gas to push it out.
Therefore when you start the car next time, the regulator pressure will jump up to normal pressure almost instantly with a couple of strokes of the pump, as the sphere is still full of oil already.
This will cause the stop light to go out immediately, although it may come back on again as soon as the suspension requests some oil to lift itself up, and then go out a second time.
On the other hand a sphere that is very low on gas but not dead means that the sphere is MOSTLY full of oil at the normal operating pressure, but still has enough gas to expel all the oil as the system pressure leaks away due to the rest of the system using it.
Then when you start up the car it will take a LONG time for the pump to pump all that oil back into the sphere.
Compare that now to a new sphere where the gas pressure is high, and the normal working condition is that there is only a moderate amount of oil in the sphere with the rest being gas, so the pump doesn't have to pump as much oil into the sphere to reach operating pressure, so it reaches operating conditions faster than the mostly flat sphere.
So to sum it up:
* Dead flat (punctured) accumulator sphere will pressurize almost immediately,
* Normal fully gassed accumulator sphere will pressurize at a slower, but normal rate
* Very flat but not dead flat sphere will take the longest time of all as the most oil has to be pumped back into it.
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">
So, how does this explain the wierd sinking when the engine starts? Where does the LHM go to allow the rear pressure to drop and the ass to sink, before the pressure can rise again and pump it back up? Does it return to the tank un-hindered, before the pump re-primes the acc sphere and returns the rear pressure to normal? Or is there some thing in the way that's meant to stop the LHM simply racing back to the tank when the anti sink valve is open.....
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Thats a good question, it shouldn't race back to the tank at all. After the car has been sitting over night the anti-sink valves will be (probably) closed, and because the height will be an inch or so too low at the back (and probably even more by the time you sit in the car) the height corrector will be already open in the direction that supplies more pressure to the suspension - but isolated by the anti-sink valve.
The only escape paths I can think of that might cause the sudden drop is either:
(a) a faulty non return valve between the pressure regulator and the rear suspension, and I'm not even sure the Xantia HAS a non return valve here. (The anti-sink valve might make it obsolete)
I know that a few older Citroen's did - for example the early model GS's didn't have this valve, while the later model GS's did have a "non return valve" between the pressure regulator and the suspension - to basically prevent the suspension back feeding to the pressure regulator if the pressure regulator had a lower pressure.
Perhaps if the anti-sink valve was faulty, and opened at too low a pressure, it might open before the regulator pressure was higher than the suspension pressure, allowing some backfeeding, but I don't see that as likely.
(b) the brake valve somehow depleting the oil in the anti sink sphere completely due to excessive leakage, such that by the time you go to start the next day, there is no stored oil pressure in the anti-sink sphere, then when the anti-sink valve opens, some oil from the rear suspension may rush from the suspension to the anti-sink sphere.
I'm not terribly convinced about either scenario to be honest, and although I've seen a few people on this message board report the exact problem you have, I don't remember seeing anyone finding a sure fire cause to the problem either.
Some things that might help to pin your problem down a bit:
(a) Is the car hydractive 2 ? If it is, the sudden drop in height could be related to the centre sphere, and a malfunction of the solenoid that controls it, or the control block itself
(b) When you say it drops suddenly, does it occur the instant you turn on the ignition ? Or just after the engine is started ? Or a few seconds after the engine is running ?
(c) Does it only happen if left to sit over night, or will it do it after (say) an hour of sitting ?
(d) From memory in one of your threads you said that the manual height control is not working correctly for the back suspension - eg going from full height to normal does not drop the rear to the right height ? If so, there is a problem with the manual override linkage where it connects to the height corrector, and its either seiezed (partially or completely) due to rust, or maladjusted.
If the height corrector linkage is sticking, its possible, just possible, that after the anti-sink valve has closed and the car has sunk a little bit that the height corrector is somehow sticking in a position that would allow it to release pressure once the anti-sink valve opens again, causing a sudden drop, with the jolt freeing up the sticky linkage to allow it to go back to correct height again.
When you pull up and get out after a drive, the back suspension presumably lifts a couple of inches due to the weight reduction, but do you see it make a correction back down to the proper height after 5-10 seconds before walking away ?
The manual height control linkage can become sticky or seized due to both road filth getting on it promoting corrosion, and lack of use - some drivers simply NEVER move the height control lever, and combined with the corrosion they can seize up.
When Dad got his Xantia (10 years old) the height lever was rather stiff, and occasionally the back would not lift from right down to normal without going to high in between. After a bit of lubrication and plenty of up/down its now fine. (Working the suspension from minimum to maximum height and back at least a couple of times a month is probably good preventative maintainance against this problem)
I'd suggest your next port of call would be in trying to free up and lubricate that rear height corrector linkage to the point where the height control lever behaves as expected all the time, and you may just find that your sudden dropping problem ends up mysteriously cured...
Regards,
Simon
So when the engine is stopped, the lack of air pressure in the duff spheres means no LHM is forced out of them. So, when the engine is started, the pump merely has to slightly top up the LHM in them (perhaps) and so, theoretically, no LHM should 'rush' anywhere when the anti sink valve opens, other than perhaps back to the reservior.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
There is an important distinction between a sphere which has *NO* gas pressure left at all (usually due to the flexible diaphram rupturing and letting oil into the "top" section of the sphere as well, and letting the remaining gas escape away through the hydraulic system to eventually bubble out of the resoviour) and a sphere that is just very low on gas but is still otherwise functioning.
Using the accumulator sphere as an example, if there is NO gas whatsoever, then the sphere is full of oil when the system is at normal operating pressure, and when the motor is off and the pressure is slowly leaking away, the pressure drops *quickly*, (since there is no compliant medium to maintain the pressure as the oil is used to maintain system leakage) but almost no oil will come out of the sphere, as there is no gas to push it out.
Therefore when you start the car next time, the regulator pressure will jump up to normal pressure almost instantly with a couple of strokes of the pump, as the sphere is still full of oil already.
This will cause the stop light to go out immediately, although it may come back on again as soon as the suspension requests some oil to lift itself up, and then go out a second time.
On the other hand a sphere that is very low on gas but not dead means that the sphere is MOSTLY full of oil at the normal operating pressure, but still has enough gas to expel all the oil as the system pressure leaks away due to the rest of the system using it.
Then when you start up the car it will take a LONG time for the pump to pump all that oil back into the sphere.
Compare that now to a new sphere where the gas pressure is high, and the normal working condition is that there is only a moderate amount of oil in the sphere with the rest being gas, so the pump doesn't have to pump as much oil into the sphere to reach operating pressure, so it reaches operating conditions faster than the mostly flat sphere.
So to sum it up:
* Dead flat (punctured) accumulator sphere will pressurize almost immediately,
* Normal fully gassed accumulator sphere will pressurize at a slower, but normal rate
* Very flat but not dead flat sphere will take the longest time of all as the most oil has to be pumped back into it.
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">
So, how does this explain the wierd sinking when the engine starts? Where does the LHM go to allow the rear pressure to drop and the ass to sink, before the pressure can rise again and pump it back up? Does it return to the tank un-hindered, before the pump re-primes the acc sphere and returns the rear pressure to normal? Or is there some thing in the way that's meant to stop the LHM simply racing back to the tank when the anti sink valve is open.....
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Thats a good question, it shouldn't race back to the tank at all. After the car has been sitting over night the anti-sink valves will be (probably) closed, and because the height will be an inch or so too low at the back (and probably even more by the time you sit in the car) the height corrector will be already open in the direction that supplies more pressure to the suspension - but isolated by the anti-sink valve.
The only escape paths I can think of that might cause the sudden drop is either:
(a) a faulty non return valve between the pressure regulator and the rear suspension, and I'm not even sure the Xantia HAS a non return valve here. (The anti-sink valve might make it obsolete)
I know that a few older Citroen's did - for example the early model GS's didn't have this valve, while the later model GS's did have a "non return valve" between the pressure regulator and the suspension - to basically prevent the suspension back feeding to the pressure regulator if the pressure regulator had a lower pressure.
Perhaps if the anti-sink valve was faulty, and opened at too low a pressure, it might open before the regulator pressure was higher than the suspension pressure, allowing some backfeeding, but I don't see that as likely.
(b) the brake valve somehow depleting the oil in the anti sink sphere completely due to excessive leakage, such that by the time you go to start the next day, there is no stored oil pressure in the anti-sink sphere, then when the anti-sink valve opens, some oil from the rear suspension may rush from the suspension to the anti-sink sphere.
I'm not terribly convinced about either scenario to be honest, and although I've seen a few people on this message board report the exact problem you have, I don't remember seeing anyone finding a sure fire cause to the problem either.
Some things that might help to pin your problem down a bit:
(a) Is the car hydractive 2 ? If it is, the sudden drop in height could be related to the centre sphere, and a malfunction of the solenoid that controls it, or the control block itself
(b) When you say it drops suddenly, does it occur the instant you turn on the ignition ? Or just after the engine is started ? Or a few seconds after the engine is running ?
(c) Does it only happen if left to sit over night, or will it do it after (say) an hour of sitting ?
(d) From memory in one of your threads you said that the manual height control is not working correctly for the back suspension - eg going from full height to normal does not drop the rear to the right height ? If so, there is a problem with the manual override linkage where it connects to the height corrector, and its either seiezed (partially or completely) due to rust, or maladjusted.
If the height corrector linkage is sticking, its possible, just possible, that after the anti-sink valve has closed and the car has sunk a little bit that the height corrector is somehow sticking in a position that would allow it to release pressure once the anti-sink valve opens again, causing a sudden drop, with the jolt freeing up the sticky linkage to allow it to go back to correct height again.
When you pull up and get out after a drive, the back suspension presumably lifts a couple of inches due to the weight reduction, but do you see it make a correction back down to the proper height after 5-10 seconds before walking away ?
The manual height control linkage can become sticky or seized due to both road filth getting on it promoting corrosion, and lack of use - some drivers simply NEVER move the height control lever, and combined with the corrosion they can seize up.
When Dad got his Xantia (10 years old) the height lever was rather stiff, and occasionally the back would not lift from right down to normal without going to high in between. After a bit of lubrication and plenty of up/down its now fine. (Working the suspension from minimum to maximum height and back at least a couple of times a month is probably good preventative maintainance against this problem)
I'd suggest your next port of call would be in trying to free up and lubricate that rear height corrector linkage to the point where the height control lever behaves as expected all the time, and you may just find that your sudden dropping problem ends up mysteriously cured...
Regards,
Simon