From experience i would say that the compressor is shot. I had similar problems on my 4.8. When i bought the car the front airbags were totally gone,when the airbags start to leak the compressor has to work a lot harder and longer just to keep the car up. The compressors burn put very easily and quickly. The accumulator pressure seems quite low and similar to the readings i was getting before i replaced the compressor. After i replaced it the readings were more like 14bar accumulator pressure. The car also jumped up very quickly after the compressor was replaced whereas before it was very slow.