Date registered: Dec 2012
Vehicle: 87 560SL, 71 280SL euro, 74 VW Ghia, 78 280Z Black Pearl, 78 280Z, 04 Corolla, 07 Odyssey, 94 Prizm
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does the car actually drop all the way to the floor?? i know some of the vehicles with full air-matic at times will just release air and drop an inch or two and stop. I would not worry too much about it unless you notice that the vehicle is completely low to the ground after it sits overnight or after a few days. If it maintains its stance relatively well, i would not worry too much about it. The hissing noise you heard is from the release valve located on the air compressor.
If in case you do have problems and the vehicle falls to the floor in the back, the most likely scenerio is a leaking air bag. In order to replace those properly, you need a lift and you need to remove the entire rear subframe to replace the air bags. Warranty time for rear air bag replacement is 5.1h for one, 5.3h for both. Also, there is a specific procedure to inflate and install the air bags. When they are not installed properly, the air bags will not last long and you will end up damaging the new air bag. This is not a DIY project. You need shop equipment and you need access to a Star Diagnosis system. Also, with your vehicle being an E500, you have SBC brakes. Those have to be deactivated when doing this repair, since if the system activates, you will throw out your caliper piston. I have seen it happen.