We used to use a short piece of plastic tubing to recirculate the fluid from the output port(s) back to the reservoir and purge as much of the air out of the piston bore as possible. Now most MC's come with a couple of plastic plugs for the output ports and the piston is cycled through a very short stroke repeatedly until bubbles stop appearing at the pressure relief port that gets uncovered at the very back end of the piston travel. Hold the cylinder in a vise and stroke the piston 1/2" or so a couple of dozen times to purge the air out of the cylinder bore, then bolt the cylinder into place and quickly connect the line(s). Then the wheel cylinders need to be bled to remove any air that gets trapped in the brake lines during the master cylinder swap. A set of "speed bleeder" screws with a one-way check valve at the wheel cylinders makes the process a one-man operation most of the time.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!