Classic/Antique Car Repair: brake bleeding, rear brakes, master cylinder

Question
I have a '68 GTO that I recently rebuilt the rear brakes and replaced the master cylinder.  I bench bled the mc, bled the front brakes and now I am not able to bleed the rear brakes. (after mkt disc on front, drum on rear)  At this point I was told that I should have gone front to rear front to rear for bleeding.  So now I am told that there is a bb in the pp valve that equalizes the pressure and that that is the problem .  So how can I reset the bb and start over?

Answer
Kevin;
Hi- first off you should have bleed from the back first, you start from the farthest  so fill the m/c and start at the l/r than r/r than l/f than r/f and keep the m/s full at all times. if you have a small amount of air in it you will not get any pedal.and bleed the m/c last front and back lines. prior to starting just crack the lines in the front and back of the valve than tighten them up, it might take some time to get all of the fluid in all of the cylds. and lines.or a bleeding tank would do wonders.