A typical hydraulic circuit is shown here in its most basic form. It consists of a tank for the oil, and a pump which draws oil out of the tank into a high pressure line.
The oil then carries on to the control valve and this can have from 2 spools up to 4 or 5. Each spool has a lever attached and these are the levers that the operator uses for hoist, tilt, sideshift, reach, etc. If the truck is engine powered the pump is connected directly to it and runs all of the time. If the hydraulic system is not needed at that time, the oil gets shunted back from a low pressure line in or on the control valve and is returned to the tank. The pump does not run all the time on electric trucks as this would cause a loss in battery life before recharging. Instead each spool/control lever has a microswitch fastened to it which cause the hydraulic pump to start when the lever is used by the operator.
The filter has to be in the circuit as the tolerances in a hydraulic circuit are extremely small and components can be ruined by even small particles of dirt. Often the filter is inside the actual tank itself. This filter needs regular servicing and this is best left to a qualified service engineer.
This picture shows a typical control valve similar to those used on fork lifts. This one is a four spool version and would be used - from left to right - hoist, reach, tilt and sideshift. This valve simply diverts oil from the main circuit to the cylinders controlling the various functions such as the lift cylinder in the mast. Any 'spare' oil supplied to the control valve is returned to the tank via a low pressure line.
It should be noted that hydraulic valves are extremely precise inside with tolerances measured in microns! For this reason they should only be repaired by qualified service personnel and even then they must be in a clean environment.
The remaining major components are cylinders without which the circuit would be pointless as these actually do the 'work'. A double acting cylinder would be used on the tilt mechanism, side shift, reach and steering.
The hoist cylinder on a fork lift only needs to act in one direction - up - to elevate the load. When the operator lowers the load the weight of the mast and load is sufficient to lower itself down without being powered. This is why there is little or no noise from the hydraulic system during lowering operations.
Finally a note about more up to date technology. Many manufacturers have now incorporated electronic control that is normally used for traction control into their hydraulic systems. When the operator pulls a lever the high level of control normally existing is further enhanced by an electronic circuit giving the operator extremely precise control over the system.