Null adjustment, or nulling the pump: Fluid from the pump inlet is tapped off through rather-coarse filter elements, passes through orifices past both ends of the spool, goes on to nozzles, and out to the return line. The orifice diameters are slightly larger than the nozzle diameters, so there is a pressure buildup at both ends of the spool. A feedback wire attached to the flapper terminates in a ball end that sits in a very close-fit slot in the spool. A sleeve around the spool can be moved left or right by a null adjustment to align the spool and body lands perfectly when the valve is first installed. (Usually null adjustment is only required at startup of the valve.)

The null adjustment usually is a hexagonal wrench fitting attached to an eccentric pin located in the sleeve slot. With the null adjustment centered, turning it one round moves the sleeve from center to full right, back to center to full left, and back to center. If the valve cannot be nulled within one rotation of the null adjustment, replace it and send it in for repair. This usually indicates a clogged orifice or nozzle controlling one end of the spool.

То есть смысл: перед установкой "spool" - золотника в желаемое положение, при котором давление с обоих сторон одиниково, рекомендуется прокачать систему (т.е. сбрость давление жидкости до нуля и стравить возможные включения воздуха.