Here's how to properly bench bleen a Brake Master Cylinder BEFORE you install it into your car...

The idea is to use two old brake lines, then to loop them back into the resevoir, submerging them into the new brake fluid

Then you slowly and repeatedly push the piston in and out. This will force the fluid through the lines and purge the air that's in the passageways of the master cylinder.

 

Hope that helps!