Hi! I have a 2011 GL450 and replaced all the air shocks. I needed to as my symptoms were like yours, starting in the front. I couldn't find the leak anywhere for a long time. Finally I sprayed soapy water on the air bag and as the vehicle was lowering and the airbag was going through the process a leak suddenly appeared. It was only visible when at a certain height. At other heights when the airbag was in other orientations the leak was too small to see. I suggest starting with soapy water on the airbags and watching all around them as the vehicle raises and lowers so you can see the airbags in all of their positions.I have a 2008 GL550 with 107K miles on it. Just this past week the right front air shock dropped when parked overnight. I checked for leaks on the air bag and found none as well as on the filler tube at the top of the shock. Intermittent problem, sometimes it will hold for days, therefore I don't believe there is a crack in the air bag itself. It pumps up fine and usually holds. I suspect the airmatic air valve distribution assembly, maybe a leaking solenoid? I have to spray down the valve body with soapy water to check for line leaks, but since it's intermittent, it might be the valve body that is failing. It appears that the valve body is located under the right front fender on the chassis member. Looks like you have to remove the inner fender well to gain access??? Is this true? Has anyone replaced this valve body due to a similar problem?
OK, just aired up system again and guess what? The air bag must have just split cuz I can hear and feel air leaking at the bottom of the bag! Time for new struts!!! Sigh...
If you need to get to the compressor, you'll have to take off the front right wheel well to access that area. I hope this helps someone else with this problem.