Here's basically how your fuel system works:
The CP3 has lift pump in the back, that is a fixed displacement gear-style, like an oil pump. This rotates at crank speed any time the engine turns. A regulator is mounted in the back near the gear pump, which closes when you apply current to it. When it closes, most all the fuel returns to the tank. When you remove current, some of the fuel goes into the 3 high pressure piston bores in the body. The crank in the CP3 pressures this fuel as it rotates at engine speed. The high pressure fuel goes out the metal line and into the rails, first the driver's rail, then a cross-over tube feeds the passenger rail (this contains a .75mm restrictor). HP fuel is now at all the injectors. When the injector gets power, it retracts the needle, and allows fuel to spray in the cylinder. When power is removed, the trapped fuel is allowed to go out the return line so the needle can return to the closed position. If one of the check balls is missing or stuck, the HP fuel can go straight into the return. This can keep the rail for any of the cylinders from reaching high pressure. Volume does not change, just where the fuel goes.