Mack Air Compressor Excessive Carbon

[mayhem1]

2007 Mack CXN613 has had at least 3 air compressors on within last 18 months. Customer says excessive carbon builds up, stops up governor, dryer, etc. I just checked cooling lines and they seem to be unrestricted. Air supply to breather seems ok but has oil residue in it. This line feeds off air filter hose between turbo and air filter housing. Wabco compressor and looks as though everything I see air related has been changed. Any ideas? Thanks in advance!