Open center. What this means is that each control valve is in series with fluid flow coming in from the output of the pump and leaving the valve when it is in a neutral position, and going on to the next valve in line. The FEL (Front End Loader) valve is normally first in line from the pump, then the flow usually will go to any auxiliary valves in line like you would normally have on the rear of your tractor, then on to the 3pt control valve and finally dumps into the "tank" ready to be passed on to the suction filter and on to the input of the pump. This flow is running all the time the engine is turning the camshaft and the pump is pumping fluid flow all the time through the circuit.
There is very little pressure rise on this circuit until the fluid is diverted to "look" into a cylinder by one of the valves, and depending on the load on the cylinder the pressure can rise to the maximum system pressure set by the relief valve. The relief valve is designed to relieve this maximum system pressure for a short period of time.
When fluid at maximum pressure flows over the relief valve a fair amount of heat is generated in the fluid and the pump is working hard. The operator is supposed to have brains enough to "center" the control valve to stop sending fluid to a cylinder that is bottomed out and let the fluid then flow at very low pressure back to the tank thru the series circuit.
The pressure is very low because in the normal instance with all valves in neutral, only the resistance of the fluid passing thru the hoses and valves causes any pressure rise at all. All systems have some losses, and some heat will be generated over time, but the engineers have taken this into account.
A lot of people think that high pressure fluid comes out of the output of the pump all the time, but that is just not true. What comes out of the pump all the time is a flow of fluid, but it is not pressurized until it meets an obstruction. It is the same for a garden hose. If you Tee'd a pressure gauge near the end of your garden hose it would read a very low pressure reading, (again there is some friction between the water and the walls of the hose), but as soon as you obstruct the end of the garden hose with say a nozzle then the pressure will rise, and if you totally restrict the flow of water the pressure will rise to the maximum system pressure.
You ask a simple question, sometimes you get a book.
