This may be caused by a dirty or clogged relief strainer from the Separator housing. You should see a 90 degree brass fitting with a clear line, inside the brass fitting is a strainer that will need to be cleaned periodically so that excess oil can easily flow back to the compressor end. You may also need to check your Air/Oil Separator filter, and your Compressor Oil filter as one of those may be clogged and causing your issue.