Take it back apart and use a 1/4" wood dowel a few inches long, cut a split in the end with a hacksaw, get some 400 grit or finer sandpaper and put through the split in the dowel you may have to double or triple it to get it to stay in, now chuck this in your die grinder or drill and polish it up. Over the years I have ran into alot of these sticking and this is what I always do when I rebuild a master cylinder, it doesn't take alot of this to do the job. Also if it is not a new piston make sure it is clean, especially in the groove that the rubber goes in. And make sure you open the tiny little hole in the bottom of the reservoir while the piston is out.
98 450 es
86 atc 200x