MG Car Repair: 1978 MGB brakes, differential switch, rear brakes

Question
I cannot get the brake fluid back to my bleeder screws. I have changed my master cylinder thinking that would take care of my problem.  I started when I could not pump my brakes up at the pedal.  I have pumped and they still do no pump up.  Where is a good place to start again with this problem.  The other answers that have been given me have helped but I need more help.  Thank you in advance for your help

Answer
Hi Jim
Your 1978 MGB should have a master cylinder with 3 pipes coming out of it.  The two in the middle each go to one front brake, and the one on the end goes to both rear brakes via the rear axle.  If you cannot get fluid to come out of ANY bleed nipple, then the problem is at the master cylinder.  

I see you have fitted a new master cylinder.  When you were bleeding the system, did you unscrew the pressure differential switch sufficiently to clear the differential piston?  It must be undone three-and-a-half turns before you start.  (Remember to tighten it again when you have finished).