I don't know how fuel or hydraulic fluid can enter the air supplied to the cabin. The usual culprit is engine oil from the No 1 bearing at the front of the compressor on a typical gas turbine. The No 1 bearing has to be well sealed as it is operating in a vacuum thus any oil leaks would result in contaminating the cabin air supply after being drawn through the compressor. I would suggest that the seals mainly leak through wear and tear.