I have a GL450 with almost 190,000 miles and have owned the vehicle for 9 years. Through all the miles I've put on it, I've replaced both front air struts and both rear air springs. About a year ago, I replaced the air compressor. Now, when I come out in the morning, the front end is down on both the driver side and the passenger side. As soon as I open the door and the vehicle "comes to," (before I turn the vehicle on), the suspension will level off so I know that there must still be air in the tank. My suspicion is the valve block right above the compressor since it's about the only thing that hasn't been replaced. Thoughts?

bump...Would like to know if/how you resolved this issue? I am experiencing something similar in my new to me GL.

It ended up being a defective driver's side air spring. Since it was an Arnott replacement, it had a lifetime warranty and was replaced. Now the passenger side is doing the same thing so I have to find that leak. It's a "do it yourself" job that can be done in a few hours in the driveway. I'm also going to replace the valve block since I have a new one. I've also recently done one of the rear air springs (easy job!). The truck now has over 200,000 miles on it and I plan to keep it until the wheels fall off! It's always something...