How to read hydraulic circuits - Hydraulic Power Pack 1. Hydraulic Pump. Hydraulic pump produces flow. Oil is pumped from the hydraulic reservoir into the system. The basic symbol for a pump: A fixed displacement pump is the simplest type and has a fixed output for each revolution of the input shaft. Modifications to this symbol describe the variable displacement pump.Hydraulic Power Packs · Output Power Basic hydraulic circuit - SlideShare Dec 22, 2012Basic hydraulic circuit. Circuit diagram: Pressure relief valve (2) As soon as the force exerted by the inlet pressure at A exceeds the opposing spring force, the valve begins to open. In this situation, for example, the piston rod is fully advanced; the entire pump delivery is flowing at the preset system pressure to the tank.