[Mgs] How to bleed master cylinder?

[Keith G.]

My '79 B (with boosted brakes and the brake pressure sensor) needs 
its rear brake circuit bled.  After not making any progress with it, 
I traced the problem back to the master cylinder.  When pumping the 
brake pedal no fluid comes out of that port on the master cylinder.

Do I need to do anything special to bleed the master cylinder?

The normal brake bleeding procedure, which has always been  
relatively easy on this car and not very time-consuming, has not 
worked at all this time.  That's what led me back to the master 
cylinder.  The front brake circuit appears to be free of air, but the 
rears are giving me a fit.

-- 
Keith G.
1979 B