There is one check valve (number 8 in Figures 10 and 8) at the end of each high pressure cylinder at the end opposite from the hydraulic cylinder. The check valve allows fresh water to enter the high pressure cylinder and high pressure water to exit the intensifier. The check valve is designed to only let water flow in one direction. Fresh water comes in though channels machined in the sides and exits through one or more holes in the face of the valve. Various seals, poppets and springs are used to maintain this water flow. Over several hundred hours these components will wear, allowing pressurized water to flow out the water inlet path, or allowing pressurized water to seep back into the high pressure cylinder. The symptoms and diagnosis of these various situations will be discussed later in the "Maintenance" chapter.