Following the discussions about the cross-flow paths and flushing, I was doing as George suggested and giving the suspension some exercise in Hard mode to clean that bit of the system out.

I had put it up to high and then removed the suspension computer fuse. I checked it was the correct fuse by ensuring that the light no longer responded to the sport switch. I then put it down to low and back up to normal.

At this point I was walking round the car and thought I would stand on the tow bar to see how much movement where was in the suspension at the rear in hard mode. It went down a little as expected. Then after a little pause the hight corrector kicked in and it started to go back up. At this point the front end lept up in the air as though it had suddenly opened the front electrovalve and dumped the contents of the front centre sphere into the system. Both front and rear then corrected to the correct height.

I managed to repeat this a second time as well!

Now, I presume it can't really have opened the valve as there was no power to the suspension - or could it? If not then what was going on?

Sticky / dry front height corrector linkages.

Hi rowanmoor, by removing the fuse the system will be in firm mode (the centre spheres will be isolated) ,inc or dec the load on any axle will cause the pressure in the suspension units to change relative to the centre sphere.ONLY when elec power is restored will the electro valve energise (soft mode) allowing pressure equalisation to take place causing the vehicle to jolt.The movement of the body over one axle will induce body movement over the other axle causing it to feel a pressure difference between it's centre spere and suspension units thereby a second height adjustment wil take place.This is normal after a power off condition ( ignition off is NOT a power off condition). I not sure of the exact sequence of power on/ power off you carried out but hope this gives you an explanation of how it happens and would refer you to the ANTI-JOLT section of the hydractive system operation which explains in more details and includes the function of the door/tailgate switches

Playing around today I noticed a foolproof way of confirming whether your front & rear electrovalves are engaging and disengaging fully.
1. Have car running after being out for drive
2. Put suspension in low, let sink for to ground.
3. Get to know the sound of your electrovalves by blipping the throttle very quickly then waiting for it to disengage about 5 secs later.
4. Keep blipping throttle to keep electrovalve closed whilst putting height lever to intermediate height.
5. Once stabilised at intermediate, stop blipping throttle and if the valves are working correctly they should disengage after 5 secs and give both front and rear a pronounced drop / low bounce as the low pressure in the centre spheres is equalised, followed by the height correctors operating to increase height again.
6. If you're feeling clever you can avoid blipping the throttle by just moving the throttle pot lever (little white plastic arm from the red box about halway up the throttle pedal) back and forth.
7 Also works by holding high pressure inside the centre spheres by doing the reverse from high to low.

odd supension behaviour, i recently repaired a brakepipe on the rear brakes (thanks willy johnstone for making a pipe up for me) and changed the rear spheres at the same time, the next day when i started out for work the car suddenly dropped at the front and rose after about 10 seconds ,then when i got to work and got out of the car there was a loud thump and it jumped up , now the suspension is like my daughters AX and i can feel evrey bump in the road ??? HELP

wirdy - I wondered that, but it was a sudden and large jump that would be very out of character for a normal front height correction. I suspect my back linkages may be a bit sticky, but the front always seems to be correct (by eye).

Robert - I still had the fuse out, so there should have been no power anywhere. That is the puzzling thing.

I guess it could have been a normal height correction mixed with slightly stick struts. I've just never noticed the struts being sticky at all - height corrections are always smooth and silent.

Terry - it sounds like your electrovalves are not working correctly which means the car is stuck in hard mode a lot of the time. Have you seen the bits about the diode fix? There is lots about it on here. I'm sure someone will have a direct link to the relevant threads soon.

OK,with the computer not powered, then the electrovalves certainly were not energised, therefore pressure equalisation (,the jolt or height correction) could only take place if internal leakage allowed fluid to return via the path of least resistance either to the reservoir or to another system component at a lower relative pressure.If your sytem generally performs well without sinking too quickly once primary hyd power is removed (the pump),and the normal ride quality is good, then it's more than likely that this leakage is acoss the electrovalves due wear or residue build up.If suspension unit wear was a factor i would expect the internal leakage to cause the body to settle quickly with no jolt.

Hi again rowanmoor , been thinking, and reckon that the behavior described with the computer powered OFF is correct.As the system cannot equalise it's pressures if the electrovalves are not energised, then any demand on any axle for a height correction will allow the only pressure source now available, (as the pump is not supplying system pressure- the engine is not running) the accumulator, to give that demand via the height correctors and electrovalves to the suspension unit causing the "jolt" as the suddern admission of fluid flows to the suspension cylinders. As the accumulator only has a finite quantity of fluid this will probably only take place once or twice as the pressure is released.The suspension will the remain firm with only the respective corner sphere supplying the springing and damping.

Ah, but I did still have the engine running - so there was a good supply of pressure.

I think it must just have been the height correctors correcting both front and rear when I didn't expect the front to, and then the front seeming to jolt either due to optical illusions or because the struts are not as free as I thought they were.