On my 2001 540 wagon, the back end would pump up when a door was opened, but after 5 minutes, the air would leak out and the back was on the ground again.
After a lot of searching, I finally found air was leaking between the two round steel solenoids and the plastic housing they sit in.
If you unplug the two electrical plugs on these solenoids, then undo the 4 philippics screws, the solenoids slide out.
Inside is a steel plunger and a spring, and on each end of this plunger is a small rubber pad which is meant to bottom out and seal the air pipe. When I tried to depress the plungers against the spring pressure, the movement was sticky and jerky, and when I looked inside, both the bore and the slider had some corrosion on them.
I used some scotch-bright pad to clean both the plunger and the bore it slides in, then I stretched the spring out to about 1" to put a little more tension on it.
I assembled the plunger with a touch of light lube and now it slides in the bore very easily. Also, clean the large orange O ring and add a little lube on it.
Now its all reassembled and installed in the car, and the air bags stay inflated......problem fixed.