Added 3 cans of freon year before last; last year needed 4 cans and system was once so empty clutch would not engage. Got 250,000 miles on this '99 ML320. Figure the seal in the compressor is leaking and the entire unit needs replacement with Chrysler/Chinese/Japanese EBay purchase. Meanwhile, I'll follow the old advice to operate it at least weekly during the cold season to keep the seal wet with oil.
What are the odds the leak is somewhere other than the compressor?