Leveret

Just to add a thought; it sounds as though the mechanics are trying to find a leak. Just as in a house, the 'pipework' almost never gives trouble because there is no wear or movement. If it does owing to poor fitment or a manufacturing fault, it's usually very early on its life. (The brake fluid pipes on my 45yr old TR7 are original). Just make sure the mechanics have thoroughly eliminated a leak at the strut airbag, especially its top O ring. You cannot detect this unless you remove the rubber bits under the bonnet on top of the strut as described in my long post above, pour in Fairy Liquid water and look for bubbles. Pointless to examine the pipes unless this has been done and a strut leak confidently eliminated.

The sagging must be caused by either a leak or a problem with the control system. The history of the problem is important. You said that it 'only seems to be when sitting a long time'. If it was the control system, the sagging would be present immediately, I think. If the level is OK for several hours, and then gradually sags, that is suggestive of a small leak gradually reducing the pressure in the strut. Have a thorough read of my post above and ask your expensive mechanics specifically if they have sprayed the strut bags with Fairy Liquid water (makes lots of giveaway bubbles) and also done the same as I did at the top of the strut housing to check the top O ring. It only takes a few minutes to do, and should have been the first thing they did.

Insufficient tyre inflation? One or more? Surely the car would think it was too low and would pump up the bag(s) rather than allow it to 'sag'? The corroded level sensor doesn't make sense either. They are flimsy looking articulated things connecting the suspension leg to the frame and it would be impossible for them to seize without breaking. Also, they don't appear to be prone to corrosion on my nearly 20yr old Audi. I would have thought if one was u/s it would have thrown up a code of some sort.

I thought I'd reply to this as apart from being hopefully amusing it might help someone with a similar air suspension problem. Although it's about my 'forever car' I think one Audi air suspension strut and compressor system is much the same as another. A few years ago my 2005 D3 SWB W12 at 70,000miles started to sag at the front when left overnight. The compressor would restore the height and the orange and green lights would go off. But the tiny leak got worse and the compressor sometimes needed two goes at it, presumably because the thermal protection switch would cut in after much noisy pumping. Lots of air was lost from the system when the clever car tried to keep itself level by transferring air from the high pressured accumulator to the lower pressure leaky strut (see Audi SSP 292, P31). If overworked, the compressor or its relay will eventually fail. To locate the leaky strut, I put it in ‘jack mode’ to prevent level equalisation. Next day only the front driver’s side had bottomed out. I removed the under bonnet trim to expose the strut's upper mounting, slid the large dust cap back along the black wire, and did the same with the smaller dust cap underneath it. With jack mode off to restore pressure in the strut, I sprayed soapy water around the strut and poured about 15mls into the top of the mounting, not filling it above where the black wire goes in. No sign of leak from the strut air bag behind the wheel but foam appeared on top of the strut mounting. Diagnosis – the tiny top ‘O’ ring has failed, impossible to replace without major work and the garage rejoicing in a big bill for a new strut. I suspect this is the most common failure rather than the larger bottom ‘O’ ring or the tough air bag itself. I replaced the foam with about 20 mls of clear water. With a magnifying glass I saw a tiny stream of bubbles appearing from below the 18mm locking nut. So I put jack mode back on and waited a couple of days till the bubbles stopped when the pressureless strut had bottomed out. Then I sucked out the water and left it to dry thoroughly. Then I injected a layer of some tough ‘CT1’ sealant. After a day to harden, I added another layer….then another. Then, jack mode off to restore pressure in the strut. Sadly, it didn’t completely cure the leak, but it took around three days to sink rather than just overnight. But a little later the WABCO compressor finally failed, rattling away uselessly. Although access is via the wheel arch for most A8 cars, it is impossible for the W12. The front bumper has to expensively come off. So I bounced it along to my capable indy to do this and the compressor, reconditioned with my repair kit from ‘bagpiping andy’, is now silent. I could live with the leaky strut, remembering to put a bespoke 30cm x 12 x 6 wooden block under the jacking point with ‘jack mode’ on if unused for a few days. It’s important to leave the car in ‘jack mode’ so the compressor doesn’t have to replenish the whole system every time the car is used. Amazingly, a few weeks ago (at 76000 miles) I noticed the car no longer sank onto the block, even after well over a week. Somehow the amazing CT1 sealant (incidentally good for repairing trainer soles!) seems to have almost sealed this top end leak! I hope this story helps someone with a similar problem as I don’t believe I’m the only one with O ring failure which a garage would probably resolve with a new strut and a hefty four figure bill.

Sadly no. And despite poking around the edges I can't see how the immovable back rest is attached. It must be removable somehow but I don't want to risk ripping it.

My 2005 D3 A8.....does anyone know an easy way to remove these for better rear visibility? There just doesn't seem to be enough headroom to get them out and I can't see how to get the backrest undone. Any advice greatly appreciated.

I beg to differ. Garages have a vested interest in advising over-cautious maintenance work. You do have something to lose if such work is carried out badly - or unnecessarily. I have read expert advice that disturbing the innards of, say, the ubiquitous ZF HP 6 box can cause filtered particles to go where they shouldn't - and cause problems soon after the unnecessary oil change. Unless it has been used harshly and arduously, it should last the life of the car. If it works, don't mend it! I would follow the advice of ZF rather than a provincial Audi garage.