I'm reviving this thread as I've developed a similar issue. Every now and then I come out after the truck has been parked overnight and the left rear is deflated. It has happened about four times over the past month. I can start the truck, it pumps back up, shut it off and it holds fine - until the next time.
So soapy water isn't going to help unless I can guess when the next time is that it's gonna leak down and even then I suspect there must be a check valve of some sort, probably in the LH and RH air valves themselves and so any leakage past that might be internal to the valve assembly and may not appear as an external leak.
Anyway, has anyone ever taken apart one of the air valves? Are new ones available? Is there anyone who has retro-fitted to coils that still has their air suspension bits in a box in the garage?
things are not as they seem - nor are they otherwise