My experience with the self leveling system on the 300TDT has been that the lever and link from the rear sway bar to the control valve has always been the problem. By loosening the lever on the sway bar and adjusting it, I was always able to repair this system.
Now that a few more years of time have passed, it could be that the accumulators have deteriorated, still, not a big deal repair. This would be a small price to pay for such a nice suspension system.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in