I have a 2010 GL450 and the rear air springs are sagging intermittently. Like maybe once a week or so. They air up fine with the car on. The left and right are both going down together. When the front ones went out it was one at a time though. Is there a difference in the rear that would make them both lose air together? Any other problems that could cause this? I have replaced the air compressor already. Thanks for any help

 

Time to replace

 

potentially could be the valve block which distributes air to the four wheels

 

One of sides is lower. Pay close attention

 

I proactively replaced both rear air springs. The next days, both sides of the rear were sagging each morning (didn't sag before the replacement). A soapy water test showed only the driver's side was leaking air where the brass fitting secures the air line into the airbag. All I had to do was tighten that one fitting and no more sagging on either side. So, not sure exactly how they are connected, but leaking on one side did cause both to sag on mine.

 

unless you're mechanically inclined and have a safe way of checking under the vehicle.
you'll have to take it to a reputable shop to have the lines and fittings inspected for leaks
worst case scenario is that you're into air spring replacement. a common thing with these trucks.
just did 4 all around with my uncle's '11 GL Bluetec.

my '08 320 CDI had 3 out of 4 replaced before i purchased it, phew...

you can get good deal from aftermarket manufacturers too
my local euro parts supplier had them in stock