Most shops use a pressure bleeder for the job, as that is the fastest way, but it is possible to gravity bleed a brake system. That involves filling the master cylinder and leaving the cap loose, and opening the bleeder valves on the calipers or wheel cylinders one at a time. Brake fluid will drip out of the bleeder valves, and the master is topped off as necessary. Letting each one lose a half cup or so would change out a quart of brake fluid, which is about what a normal service does.