GL 450 with weird air suspension problems
 

My friend has a 2007 GL 450. I'm trying to figure out what causes the air suspension to lose height while the car is parked and the engine off. After a few hours of being parked, the whole car seems to sink a few inches. The passenger rear corner seems to sink deeper than the others. After you start the engine, it self-corrects. Typically what is the problem? Is it a module? Are all four-shocks leaking?

if the entire car sinks down in just a few hours usually it is a valve block issue. if it is leaning to any one particular corner usually that air bag is bad. 164x has air springs in the rear and air struts in the front.

Easy way to check-put the vehicle at its highest ride setting (raise it), spray all the air bags with a dish soap and water solution, watch for bubbles. The rubber actually dry rots just like a 8-10 year old tire, they crack and leak right in the fold of the rubber bladder. If you don't find a leak in the rubber bags check the valve block with said solution, but 99% of the time the bags are leaking.