If you're reluctant to drill the hole (as I am), try this:
remove the slave from the bell housing, being careful not to let the rod come too far out. You must not allow the piston to get out of the bore, but it should come out further when off the vehicle and still be safe. It should more or less relax at that maximum point, but take care anyway. Make sure there's plenty of fluid in the reservoir. Now compress the piston as far as it will go, and slowly let it back out, again being sure that it doesn't come out past the bore. Try doing this a couple of times, and it should bleed, because, in theory at least, the slave piston actually displaces more fluid than the line. The Centrix replacement units also come with no bleeder, and this is what they recommend, but they also note that it may take a number of clutch actuations before it firms up. Both my son and I have found that done this way it does firm up and stay good, if all else is good.