Fairly easy to do,squeeze lever to reveal pivot clamp grub scew and loosen to remove pivot pin.Remove lever from bars, screw adjuster rod through barrel(be careful to get the allen key in straight, the ends split very easily) and place lever blade to one side.Easiest now to clamp the body in a soft jawed vice and using J-0 size circlip pliers remove the circlip.The piston is sprung so it will come out far enough to pull it out on it's own.A smear of silicon grease on the new piston assembly(remember to fit the spring)and slide it back in, washer on the operating rod,circlip on on pliers and allen key into operating rod to press the assembly into place while you lock the circlip back in.Re-fit lever blade, fit on to bars and bleed.