Hy Guys, an old GM engineer who's name escapes me at the moment said "parts left out cost nothing, and cause no service problems, so you can be sure if we put it there it's there to do a job". All drum brake systems I've ever worked on have had residual pressure valves, they keep the cups against the wheel cylinder bores to help prevent air/moisture from being drawn into the system when you release the brake pedal, and they keep the system "charged" so your first application actually applies the brakes, rather than repriming the system. I'm not an engineer, but I do repect the work they do, and I'm not about to try and out think them.
