A hydraulic system uses hydraulic fluid or tractor fluid to operate machinery. Pressure is exerted on the hydraulic fluid as it passes through small hoses. The force exerted by this pressure on the fluid drives the machinery. A hydraulic system utilizes a variety of valves and tubes to push the hydraulic fluid through the machine. A hydraulic pilot valve is the part of the machinery that controls the high pressure of the hydraulic fluid as it passes through the machine, and regulates the functioning of the other valves.

Valves in hydraulic equipment are commonly called pilot-operated valves. These valves can be pressure regulator valves, solenoid valves or check valves. The pilot valve acts as an open and close switch which allows for the passing of hydraulic fluid into the other valves. Once the fluid reaches the other valves, each valve completes another part of the hydraulic process to insure proper operation.

Pilot valves are commonly two or three port valves and have a poppet or sliding design. A poppet design is simply a disc with an opening that opens and closes. The sliding or spool design utilizes a metal shaft and spring. As pressure builds up on the shaft it pushes the springs and opens the valve. Poppet designed pilot valves are considered to be direct-acting valves since there is no pressure minimum required to open the valve. Spool or sliding designed valves are considered to be indirect-acting valves since there must be a certain amount of pressure before the spool moves.

Hydraulic pilot valves have a predetermined pressure setting that dictates when the pilot valve opens and closes. As pressure builds up around the pilot valve, the pressure sensor determines when the valve should open. The pilot valve will then release hydraulic fluid into the other valves until the pressure setting is below the maximum pressure. The secondary valves are completely dependent on the pilot valve. If the pilot breaks or malfunctions in any way, the entire hydraulic system becomes unusable.