I thought I'd post this story as apart from being hopefully amusing it might help someone with air suspension. Although it's about my 'forever car' A8, I suppose one air suspension strut and compressor 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 accumulator to the leaky strut (see Audi SSP 292, P31). If overworked, the compressor or its relay might fail.

To locate the leaky strut, I used ‘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 suspension mount, 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 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 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 could see 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 some time 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 thoroughly 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!