I can confirm what Eric has said about their being residual oil / emulsion in  the bottom  of the C-drive.  I use a 60 ml syringe irrigating syringe with  a short piece of tubing inserted thru the drain plug hole to suck this residual solution out  of this are.  The tubing usually has a natural curve to it from being coiled and will go naturally into the lowest part of the C-drive.  By cutting the turning on a bevel on the lower portion  of the  tube I can usually suck out at least 120 ml of residual solution.  I then use solvent to flush the area using the same tubing and again aspirate as much as I can.  Lastly,   I use a surgical "tail sponge"  inserted thru the drain plug hole to "dry" this "sump" area.  A long piece of lint free material would serve the same purpose.

Gary S. Silver, M.D.
s/v Liahona
on the hard in Puerto Del Rey Marina,  Puerto Rico