Rear Air Spring Replacement - with photos

[Davee]

I have a X5 2002 E53 and recently noticed the rear air suspension problem.
After not driving the car for a week noticed the rear suspension had lowered (both sides).
Started the compressor pumped up and was fine for a few days.
Then parked the car outside - frost overnight the suspension lowered again (Both Sides).
Started the compressor pumped up and was fine for a few days.
Next morning the suspension was fine however just parked the car after lunch,, on my return after work the suspension had lowered (both sides).

While i'm 80% positive I need to replace the air suspension the doubt I have is why both side failing at the same time.

What are the other common, single point of failure in the system that could cause this. Is there a signal valve somewhere that feeds both sides that can be checked?