Use 10 lb valves for drum brakes (if they're not incorporated into the master cylinder) and 2 lb valves for disc brakes.
The misconception that the purpose of residual valves for drum brakes is to keep the shoes against the drum is a common one, but I'm very surprised to hear that Master Power's site is repeating it. They know better.
It takes around 70-80 lbs of fluid pressure to overcome the tension of the brake shoe return springs. There's no way that residual valves can even begin to affect the position of the shoes. That's determined by the regular shoe adjustment procedure.
The residual valves prevent air from being pumped around the wheel cylinder cups by the action of the master cylinder return spring during brake release. That's all they do.
In disc brakes, the 2 lb valves prevent siphoning from calipers to master cylinder. They are most important when the master is mounted low, as under the floorboard in a stovebolt.
The type of friction material is irrelevant to the residual valve question.
Joe
