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!!!!