... closed-center system, fluid is under pressure throughout the system when the hydraulic pump is operating. Both systems are discussed in the paragraphs that follow. Open-Center System. Figure 2-7 shows ... flow. The most common power-driven pump is the piston pump. In all but the simplest hydraulic systems, variable-delivery pumps are used. A variable-delivery pump delivers only the amount of ... pressure. Hydraulic motors perform a variety of functions, including raising and lowering cargo doors, operating rescue hoists, and positioning wing flaps. 17 AL0926 ...