Many tractors, though, use a simple gear type pump. The oil comes in the larger of the two openings, enters the gear tooth cavity, and then is expelled when the gears mesh, thus forcing oil into a smaller high-pressure line. The tolerances are very close on these pumps and it is not wise to disassemble one unless the tools and gauges are present to do adequate service. After the oil is exhausted from the pump it travels through a high-pressure supply line to a spool valve. A spool valve is a fancy name for a switch that controls fluids.
Oil is directed through the spool valve to the selected circuit of choice.
If nothing is selected then it flows through the valve back to the reservoir. When a lever is actuated the oil flows through the valve and into the now opened circuit of choice, usually to a hydraulic cylinder performing some function. As the oil flows under pressure to one side of the selected cylinder the ram extends or retracts depending on the selection. Oil from the other side of the cylinder flows back through its high-pressure hose, through the spool valve and back to reservoir. When the spool-actuating lever is pulled the other direction the process is reversed and the hydraulic cylinder actuates in the opposite direction.