Hi Darren,
First, run a scan and see what that reveals. There can be reasons for strut collapse other than a ruptured air bag. If the problem is definitely a leaking bag, what follows will be pertinent.
So, with more interest in C6 air suspension parts emerging, I'll put up the info here for anyone who might be facing impending failure.
Audi only sells a few hundred allroads a year in Australia, so there isn't any non-factory aftermarket parts support to speak of when it comes to air suspension. Thus, my research and reason for buying from Germany is based on comparison with Australian premium pricing. It is fair to say that whether it's computer software, or anything automotive, both the cars themselves and spares, we are gouged in this country. It may be different for those of you in the UK or Europe.
I went to
www.ebay.de and found Air Suspension Shop selling
refurbished struts. A listing for a right strut was there a week or so ago at
http://www.ebay.de/itm/Audi-A6-C6-4F...item5d4286b696
When I looked last, there was a note to say there were none available until January 2014.
In your case, enter Audi A6 C6 4F Luftfeder Luftfederung 4F0616001 Niveauregulierung in the ebay.de search panel and you'll find the same seller with rear units on sale.
I looked for his website and found it at:
http://www.air-suspension-shop.com
The same rear bag is EUR20 cheaper buying directly from the factory.
After I receive the EUR400 deposit back when I send him my dead front struts, I will have been able to replace both sides for the price of one new side had I bought from Audi in Australia. No core deposit is required for the rears.
While the front end is apart, I'm replacing control arms as well, with the rubber bushes showing signs of age.
For these, I have found Rexbo in Germany well-priced. I have bought parts for my other cars there, BMW among them, and use them for their very reasonable freight charges (the tyranny of distance) and for the wide choice of OEM parts. I am happy with Febi bits but have no problem with Lemförder or TRW.
Go here for rexbo:
www.rexbo.de
For those not domiciled in an EU country, both suppliers remove the German VAT, (also called MwSt or USt) for export sales, thus reducing prices by 19%.
For the brave, there are myriad sellers of almost anything Audi in China.
Audi A6 Air Suspension Air Spring, Audi A6 Air Suspension Air Spring Products, Audi A6 Air Suspension Air Spring Suppliers and Manufacturers at Alibaba.com
I priced my front strut with one and they offered the part at USD400 with USD160 shipping to Australia. I know Audi does manufacture there, however...
Sorry to say I have no experience with rear bags but suspect that soapy water may be your best friend assuming your bag can retain pressure long enough for it to be able to exhibit a leak. What I would do if I suspected a leak in the rear would be to paint the whole unit with a thick-ish dish washing detergent mix with the strut still holding air and look hard and long for bubbles. Apply it to any runs of air line and associated connectors while you are under there.
I know ozone is bad for rubber and maybe proximity to the sea is as bad for air bag material as it used to be for tyres before the chemists added some magic ingredient. Just a thought.
Regards,
oz.