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?

Just run thru the test procedure in SDS and it will point you in the right direction.

Unfortunately, I don’t has access to SDS.

That sucks. You could probably try spraying soapy water on whatever lines and connections you can access. You are really going to need SDS to really diagnose it for sure as it pumps it up and is able to measure leakages etc in the different segments of the sytem and will pretty much tell you which branch to look for leaks, such as if the leak is the compressor, between compressor and valve block, valve block to struts etc.

Thanks. Since I replaced the compressor in the last year or so and it worked fine, I'll assume it's not that. Both front struts have been done in the last 3 years. Since the vehicle isn't leaning down on any of the corners but on the front end as a whole, I figured that the only thing/connection the front struts have in common is the valve block.