You have to close the bleed valve. If it's closed, is the rubber in the brake fluid reservoir taking up air space? Make sure there's no extra air in that box. It works like a vacuum.
If that doesn't work, drain all the brake fluid, and blow compressed air into the piston. Use a block of wood, like a shim, between the pads to keep the pads from closing all the way. Good time, then, to clean your brakes.