some moisture got in the master cylinder from a loose seal. crud in the oil traveled to the slave cylinder piston cup and left more crud. i cleaned the piston and bore with 1200 grit to remove the tarnish and more crud, also cleaned the master cylinder (take it all apart) be careful of the spring and seals there. replace with dot 4 and bleed with brake vacuum pump slowly from the bleeder pipe. it took several refills, but be patient to get all the air out. total time for r & r 1-1/2 to 2 hours. spent more time cleaning up.
ps. don't forget to add some grease to your clutch push rod while your there.
