Apologies for butting in but I have a related problem.....
Over the last 18 months the rear suspension (non full AIRMATIC) of my 2006 S211 Estate T model very often (but not always) sags overnight on the near side but sometimes on both sides. But still the compressor kicks in and pumps it up on the next morning start. Even today.
So I spoke to my inde back then and he said it was very difficult for him to find suspension problems when sometimes it was ok and sometimes it wasn't. "Leave it 'till it is permanently down and bring it in". So I left it and, to date, it has never stayed permanently down - the compressor still pumps it up and and I have only had two occasions during this 18 months when the red message 'STOP -car too low' appeared on the dash.
Some days there is no sag and some days both rear suspension units are low. I use it every day for short 15 min return journey (dog walking) and every day the compressor kicks in for 20-30 seconds - no red dash message. The last 'too low' message came up after a 650 mile journey last October - and went quickly as usual

when I re-started. Drove home last November from southern France with no issues reported on the dash - but after stopping on the autoroute for 10 mins the near-side had definitely dropped in relation to the off-side - no 'too low' message despite being holiday-loaded when I re-started

.
This evening, after today's' dog walk, I measured the distance from the ground, thru' the wheel centre to the bottom of the wheel arch. Result: off-side 64 cms, near-side 61 cms, so the near-side is down in relation to the off-side after 4-5 hours. What should this measurement be on a healthy system?
So I am confused....Shouldn't the compressor have given up the ghost by now if it was being overloaded? If there is an air leak in the air spring surely the high pressure would cause it to deflate rapidly every time (it doesn't)? Are their any valves to go wrong that dump air from the suspension? You would have thought 'yes' because if the compressor pumps up the system under heavy load then take the load out and the system would need to compensate? Ride-height sensor or what? Could it be anything other than the air springs causing the problem? Would STAR be able to identify suspension issues? I am at the point that I will replace the air springs to avoid killing the compressor otherwise. It's been happening for 18 months after all! TIA