Don't know much about the 500SEL self levelling, but if it's anything like the W124 E-class then it sounds lke your gas spheres are shot. I expect the ride will become harsh again in a few days. The air you bled out will be what should be trapped behind the now ruptured diaphragm.

W124s use ZH-M fluid (part No 000 989 91 03) in the suspension, which is different from the steering (000 989 88 03), not sure about yours, but if you are changing the spheres you'll be replacing the fluid anyway.

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Mick J
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)