Drum brake master cylinder for power brakes (p/n 5460465) has dual bleed valves, one for front and one for rear, due to the master cylinder being mounted at an incline when bolted to the booster. The correct master cylinder (if configured / rebuilt properly) also has RPV's (Residual Pressure Valves - a type of check valve) internally at each connection behind the seats. The RPV's prevent pressure bleed down back into the master cylinder and keep the shoes properly positioned. No proportioning valve needed.
Attached link has some good information and used to have some attachments which for some reason are no longer there.
Maybe Kurt or one of the other moderators can address the missing attachments.http://www.camaros.org/forum/index.php?topic=5983.0