Devilmans right! So ... loosen the bleed nipple. Clean, lubricate the piston with fresh fluid, and re-insert the piston. Push it right back. You could, to minimise mess, put a bleed pipe on the nipple, other end into a jar, to catch the fluid you're going to displace when doing this. Fit the cover again, then bleed up the system.
And welcome to OSS.