There is a second reason for having residual valves on drum brakes. The small amount of residual pressure keeps the cups in the wheel cylinders expanded. This prevents air from being drawn into the system. Some later drums have expanders on the internal wheel cylinder springs that eliminate this problem.

Here are some links:

Typical Brake System Configurations

Camaro Brake Valves

Classic Chevy, Chevrolet, GMC, Ford technical articles

Hot Rodder's Bible : The Ultimate Guide to Building Your Dream Machine - Steve Hendrickson Gerry Burger - Google Books

Brake Systems: OEM & Racing Brake Technology - Mike Mavrigian, Larry W. Carley - Google Books

Designing Your Brake System