Here you can see the basic elements of a typical hydraulic system. The hydraulic oil is drawn from an open pan through a filter and sent to the control unit at the top. Between the pan and the control unit is the junction to the pressure relief valve, which if overloaded, is protected through the spring-adjustable maximum pressure of the pump and pump-drive.
We have consciously selected a hydraulic control unit, to show that it doesn't always have to be electronics. Before the appearance of electronic controls, the hydraulics had, e.g., played a significant role in the regulation of the shifting operations in automatic gearboxes.
At this point (see picture 1), the plunger with the two pistons is thus positioned that the pressure is applied to the left side of the working piston. If it still has momentum to the left, then this is slowed down and very soon the movement to the right is initiated. Through a mechanism between the working- and the control unit, it will carry out the first part of the shifting by itself (see picture 2).
The control unit is then also moved, for as long as it takes until its line on the above right is opened, thereby changing the direction of the working piston once more. All together, in this implied example of a complete hydraulic system, it comes to a relatively steady back-and-forth movement. That this regulation could be, perhaps with less effort, electrically realised, is beyond doubt.
One further fundamental observation: What we have here, is an open system, this has nothing to do with whether the oil-pan is open or in fact, hermetically sealed. The difference between the two systems lies more in the pressure drop in the open system. Thus, the pump has to build up the pressure over and over again. This is not the case in a closed system. The pressure remains constant because it is in circulation. 05/12