The residual check valve maintains between 20 and 60 psi in the brake lines.
Residual check valves hold pressure from 2-15 psi and can be used in both disc and drum brake systems. This is an average of course. A manufacturer might have a reason to increase this, but 2-15 is the norm. It sounds as though you are enquiring about a drum brake system. In this case, the residual check valve is meant to hold pressure on the seals in the wheel cylinders to keep them seals. If this wasn't done, the seals would relax and brake fluid would seep past and leak.
As for how the shoes of a drum brake system create stopping force, you are partially on the right track. The exception is centrifugal force is the outward force applied to an object moving from the center line of the spinning object. It is true brake shoes are inside a spinning drum and there is certainly some centrifugal energy at play, but centrifugal force does not help the shoes apply braking force. The spinning of the drum applies a lever or wedging affect to the two shoes. The primary shoe tends to be the shoe that experiences the most force when this occurs. The primary shoe is always mounted toward the front of the vehicle.
It's important to note, there are more than one type of drum brake system, which I won't go into here. Other systems don't use both shoes acting on each other to produce a wedging effect. They work more independently of each other.
You asked for a true or false answer. To which my answer is, as soon as I say false, there will be a car or situation that is true and vice versa. When I work on a car, the ideas and decisions made are about that specific car. Manufacturers are always trying new things and as soon as you decide something is absolute, your going to find yourself having a hard time wrapping your head around something that doesn't fit with your ridged idea of what should be.
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