The proper way to do this job is to remove all the internal parts except for the tumbler. Be sure to secure the fly before going any futrher. Next, open the jaws of your vice just enough for the tumbler to fit between them. Make a punch that just fits on TOP of the tumbler square, not down in the screw hole. With the lock on top of the vice jaws and the punch set on top of the square part of the tumbler, give the punch a moderatley sharp rap with a hammer. This will drive the tumbler square out of the cock or hammer without any damage. Again, DO NOT try to pry the two parts with a screwdriver. DO NOT put a punch down in the bottom of the screw hole. And, finally those devices that act similar to a gear puller that are sold by some companies are really no better than using a screw driver under the cock, I do not recommend them.