Unfortunately, filling it won't solve your problem. There's air in there. Your clutch won't even release fully. Its a hydraulic system just like your brakes. Before you waste your time and brake fluid...

replace the slave cylinder
replace the master
replace the flexible hose

get some aquarium tubing from anywhere, heat up the end so that it slips over the nipple on the slave. Crack it. Fill the master. When you start seeing brake fluid in the tubing, then route the tubing up to the master so that you have a closed circuit. Keep pumping until there are no more air bubbles. Close the bleeder, make sure your clutch disengages. No? tap the master/slave with the big end of a screw driver to displace air bubbles. Rinse, repeat. When its done, close the bleeder, remove the tube, have a beer.