you may need to use a pressure bleeder(such as gunsons eazibleed) but before buying one, try opening bleedscrew & allowing the fluid to run down, just using gravity & not pumping ( i'm assuming you've tried the pumping method). another idea is, if you have good access to the slave cylinder, open the bleed screw & push the piston in with your finger, tighten bleed screw & release, keep repeating this several times.
jim
January 2010