OLL99 and All,
Sorry to hijack this thread, but I have a very similar problem.
My drivers side rear drops right the way down after a few hours. The front stays up fine and the passenger rear side drops slightly (I assume trying to compensate for the other side.)
When I start the car up, the suspension does return to the correct height in reasonable time, but slower than usual and therefore I would guess that it is either having to refill the bag or the reservoir or both.
I still need to check whether the compressor switches itself off when enough air is in the system or runs continuously (trying to replace leaking air)
I also need to do a more thorough soapy water check, but so far the valve block area hasn't thrown up any leaks.
As the car takes a few hours (2-4) to drop, I am assuming that I have a slowish leak.
My thinking is that the non-return valves, valve block and reservoir are ok and the problem is either a drivers side:-
1) leaking rear air bag.
2) leaking rear air line.
3) faulty rear height sensor.
Reading the previous replies, I think I am on the right lines!!
Aren't I?