I wouldn't run air through a hydraulic system, even if you have driers and a filter you still have condensation issues. I got an open quick attach fitting for the end of one of the implement hoses, then I hook the other hose to my tractor and turn on the flow briefly and catch what comes out in a 5gal bucket. You can probably see the change in color, or estimate the volume. Of course you then need to top off your own tractor if you purge very much.