Put some fresh fluid in the receptical to prevent inadvertantly getting air into the system if the brake pumper happens to let up on the brakes when the system is open.
BTW, the resident brake expert at a big Mercedes dealership told me they stopped having to replace master cylinders when they stopped using the old fashioned pumping method. He said that a surprising number were being damaged. The key is to not allow the brake pedal to go beyond where it normally goes, i.e. do the bleeding in small increments.