This morning I decided to swap wheels and tyres on the 690SEL to try and find out where a 'tap tap tap' noise came from (it had developed after the day at Oran Park in November).
I was almost sure it was tyre related as it almost disappeared at higher speed (or it was drowned in road noise).
I did the usual hydraulic lock (centre position) on the dash and proceeded to replace the rear wheels first as I thought they were the cause of the problems (see pics of 690SEL on three wheels available on other thread relating to track day)
However, upon bringing the car back to the ground the rear end refused to come down and no amount of coercing (lifting it up and down on many occasions and keeping the controls in all different positions for a total of three hours) would bring it down from its fully extended positions.
I even drove it around the block and it felt as if it needed new spheres at the back (replaced all spheres in August).
AFAIK, this can happen under certain conditions where the valve becomes hydraulically locked. It sounds like the system was still pressurised when you lifted it, filling the fully extended struts.
Did you set the main valve to the service position with the engine running prior to lifting the rear?
I would actually depressurise the central sphere and see what happens.
I think if you disconnect the cable at the distribution valve and swing the plate all the way back its supposed to de-pressurise the system (which is why its can't be moved to that position from the dash control... wouldn't be a good look the car sinking while travelling along the highway etc.)
You have managed to get more pressure into the circuit between the rear levelling valve and the rear struts than there is in the circuit between the distribution valve and rear levelling valve. This causes the rear valve to lock up and refuse to discharge. I have had this on both the front and rear of my car at various times. I'm not sure what the proper procedure is, but I supported the car on stands, and drained off some of the pressure from the rear bleed screw to lower the rear circuit pressure below the system feed pressure again. You need to work with caution - the fluid comes out at ridiculous pressure.
S class is right and you will need to support the car and release the pressure in the rear circuit.
If you have a later car with a bleed valve at the rear all you will have to do is crack it open with the car lifted and supported and once the pressure is out tighten it up and all should work as expected.
If you don't have a bleed valve then crack one of the pressure lines and it will leak out a small amount of hydraulic fluid under pressure and then should behave itself again.
Just don't get sprayed by high pressure fluid or drop the car on yourself.
Alabassi had the same problem with a 6.9 he was repairing last year and he cracked open a line and the rear suddenly deflated and then worked normally.
Lifting the car with the lever in the middle position will keep pressure in the system which you do not want.
You can either lift all four wheels off the ground with the lever in the normal position which should depressurise the system sufficiently or if you want to be sure disconnect the cable and move to the "M" position (which removes the pressure from the central sphere but there won't be much in the systen anyway after a straight lift) - then depressurise the rear by loosening a line or the bleed valve.
When the pressure has been released and the coupling or bleed valve have been tightened again, lower the car so that the car is supported by the lifting device but the wheels are on the ground at or around normal ride height then start the engine and let the hydraulics pump up to normal ride height.
All should be well and so long as you have not been penetrated by flying high pressure oil all you will have lost is a bit of time and a few tablespoons of oil.
P.S. Checked in the Workshop manual and front and rear vent (bleed) nipples were fitted after chassis no 1985 so your car should have them.
10/4 - Summernats is strange here - all the locals avoid it, while lots of people come from interstate just for it (and to buy some "adult goods" while they're here).
Apparently it is also the busiest few days for Woolworth in Dickson (the closest supermarket to Summernats) for the year - they sell lots and lots and lots of sausages, coke and something strange like scissors too.
I have had the 690SEL in the air and opened the nipple at the juction near the rear (passenger side) aircell.
I have let out almost a cup and a half of fluid and then lowered the car and there was NO joy with the pressure still on and the suspension fully extended at the back.
I also started it and moved the disc in all positions without any effect on the back (the front went up and then up again when the disc was turned all the way).