It doesn't have to be bled on a bench per say. I always have it mounted in the car and ready to go. Less mess than trying to bleed it in a vice and then trying to install it in the car spilling brake fluid everywhere. Just hook up the fittings and tubes to the master and run the tubes back into the resivior always make sure they are submerged in fluid. Pump the pedal slowly till the air bubbles are gone. The goal is to eliminate all the air trapped in the pistons inside the master. FWIW I have installed brand new masters that leaked past the pistons and did not build any pressure. Never assume.
Manual Brake master cylinders are pretty level but the power brake master cylinder has a pretty good angle to it. That is were air can get trapped. For that reason is why they recommend to bench bleed it. That way it is level an no air get trapped.
