After driving my firebird, Ive determined the brakes needed some attention. so we bled the brakes, after a month went by the brakes felt squishy, so we bled again, after going thru this a few more times, we determined to go a little deeper into it and replaced the master cylinder, all brake lines, 2 proportioning valves and brake booster. we are getting about 15psi of vacuum(b/c of heavier cam). we get good pressure on the front brakes(disc) but the rear brakes(drum) have no pressure or fluid that comes out. please help!!!!

disconnect hose at rear brake valve (proportion) to see if fluid is able to be pressurize by master cylinder. Then check after the valve. Have you bench bleed master cylinder?

After the master is bench bled and all the air is out have an extra person hold in the button on the proportion valve while you bleed the rear brakes.