Hard-closed GV5, soft-closed GV6, isolated IP5, connected aux. pump cart to GV5 annulus (did not valve-in), spun-up YBM MTP -> all in preparation of venting YBM -> Did not vent YBM as scheduled.  Had shut down purge air compressor in the morning to inspect internals of airbox and ended up spending the rest of the day disassembling the airbox, cleaning and caulking leaks noticed between the sheet metal stitch welds which were allowing compressor cabinet air to enter the compressor downstream of the inlet air filter.  The compressor crankcase vents inside of the cabinet and this air is laden with turbine oil vapor.  This air was entering the 1st stage compressor inlet by way of small unsealed passages (gaps) which occur between the welded sheet metal airbox stitch welds. Inlet air is intended to enter from LVEA ducting only.  Evidence of condensed oil was noticed in the intercooler condensate drain bowl but nowhere else downstream -> I showed John W. the details of my concern.  In addition to caulking, I fabricated gaskets for the bolted sheet metal components of the airbox.  These steps are expected to eliminate cabinet air from entering the compressor intake.