The instant you disconnect the lines from your PM, you introduce air into the lines. Bleeding is the ONLY way to fix that. Besides, with a NEW Master cylinder, you should also put new fluid through out the whole system.
Bench bleeding the MC is a method of FULLY compressing the piston even FARTHER than the brake pedal can compress it. Otherwise you will LITERALLY never get rid of the air in the MC.
Do it right, bleed all 4. Brakes are NOT something to take short cuts on.
With that said here is the procedure to 'bench bleed' a Master Cylinder:
ASSUMING YOU ALREADY HAVE THE TUBES AND FITTINGS IN PLACE
The QUICKEST AND EASIEST way to bench bleed it is to use a clamp. If you have a 'quick clamp' like the one shown here:
put a small enough socket so that it fits INSIDE the bore, but large enough to be stable. Now 'clamp down' until you bottom out the piston, but DON'T FORCE ANY MORE RESSURE. Add fluid. There is a switch on that particular clamp that allows to release pressure at the same rate you apply pressure. Release some clamping force (3-4 squeezes). Repeat this process of applying and releasing pressure until you produce NO MORE AIR BUBBLES through the tubes that go up into the resivior (sp?).
3 min. TOPS to do it right.
Ryan