has fluid flow but no pressure until some hydraulic device is operated. When the selector valves are
OFF, fluid flows from the reservoir to the pump through the open-center passage of each valve, then
back to the reservoir. No restrictions exist in the system; therefore, no pressure is present. When one
valve is placed in the operating position, a restriction is created by the device the valve controls. Fluid
then flows under pressure to that hydraulic device.
Closed-Center System. Figure 2-8 shows a basic closed-center system. Fluid is under pressure
throughout a closed-center system when the pump is operating. When the selector valves are in the OFF
position, fluid cannot flow through the closed centers. This causes pressure to build in the system; it is
available at any time a selector valve is turned on. A relief valve is used to keep system pressure from
going above a predetermined amount when all valves are off.
Figure 2-8. Basic Closed-Center System.