Chevrolet Repair: Brake problem, chevy c, drum brakes

Question
1974 Chevy C-10 pickup; 350 w/auto & power brakes - disk front, drum rear.  have replaced power boost unit and master cylinder but brakes not working properly.  Cannot bleed back brakes - pedel will not depress when bleed valve open.  Can it be a failure of the pressure compensation unit?  I have run out of ideas and things to replace.  Thanks, don

Answer
Hello Don,
I assume you bench bled the master cylinder before installing it?

I am sure you have a proportioning, or metering valve to differentiate the pressure between the disc and drum brakes, so cracking a line to see if there is pressure, or rather where there isn't pressure may be what you need to do.

I will send a link that you will find very helpful.

www.classicperform.com/TechBook/BrakeTroubleshoot.htm

Good luck, and get back to me if that doesn't help.
I HAVE installed new master cylinders that didn't work, but don't just jump there.

Van