how to bleed master cylinder?

[John Larson]

I find it of great benefit to bleed the master cylinder prior to bleeding
the system.  I spread a few rags below it and leave the lines loose as I
fill it, and even leave them open for a while while I do other things.  I
then close them up and have a helper pump and hold the pedal until each
fitting passes only fluid when cracked.  I then bleed the calipers.  This
way you're not pushing (or pulling) all that air through the whole system.
Cuts the overall time way down, makes the job easier, and aids in getting a
really good pedal feel.  HTH, John