FWIW, the 10 PSI residual pressure valves do not provide enough pressure to resist the force provided by the main return spring on the shoes. Doing the math, 10 PSI across the piston area of a Ø15/16" bore wheel cylinder, like the ones my '77 runs, would equate to about 10.7 pounds of force, as compared to the relatively huge force provided by the return spring (60 lb/in?...80lb/in?...whatever it is, it's a lot). Force = Pressure*Area, where the area of the wheel cylinder is .69 inches squared and the pressure is defined by the RPV as 10 lb/inches squared.
My understanding of the purpose of running a 10 PSI residual pressure valve in a drum brake circuit is to maintain pressure on the cup seals inside the wheel cylinders when the brakes are released and the return spring collapses the wheel cylinder and allows the shoes to disengage the drum. By maintaining some pressure on the seals, they remain "inflated" and in contact with the bore of the wheel cylinder, preventing leaks of both fluid out from and/or air into the system. Without the residual pressure, the original seals were prone to "burping" fluid out and air into the system, leading to a lower and lower pedal over time with less and less rear brake contribution, hence the solution of the valve. Most OEM's don't add anything to a vehicle unless needed to address a specific problem.
That said, modern day material advances with the polymeric wheel cylinder seals have all but eliminated the need for the 10 PSI RPV's, which you may notice when purchasing modern master cylinders for older applications, as many will be provided without an RPV in the drum brake circuit outlet port. Back in the day, there were some mechanical solutions as well with spring expanders and such, so not all drum brake applications used the same RPV solution.
As ntsqd noted above, 2 PSI valves are great for low-mounted master cylinders (race cars and cars/trucks from the early 60's or older) as well as off-road applications where wheels are regularly above the MC height. Wilwood added a 4 PSI option a few years ago, and it serves as a good option to combat pad knockback, something more commonly associated with track-day type applications, but is much easier to implement than individual springs under the caliper pistons to achieve something similar. And then the 10 PSI units, which have become less and less of a requirement in many modern applications, but certainly won't hurt anything in a drum brake circuit.
Tobin
