This type of level control valve is a hydraulically controlled diaphragm-operated automatic control valve that closes to a preset high tank level and opens in response to the level drop, keeping the tank full. The water level is maintained by the use of a high precision pilot level control valve that accurately detects the height of the water level from a detection point at the bottom of the tank and controls the main fill valve accordingly. This eliminates the need to install a flotation mechanism, which makes it ideal for high level reservoirs.
The pilot valve senses the static head of the reservoir level via a sensing tube  connected to a “still point” at the bottom of the reservoir.
- When the height of the water level increases to match the setting of the pilot valve , the pilot valve will close, therefore allowing the pressure of the inlet water of the valve to accumulate in the control chamber of the main valve  through the adjustable needle valve , causing the main valve to seal tightly.
- When the water level head drops below the pilot valve setting, the pilot valve opens to release pressure from the pilot valve. causing the main valve to open and fill the tank..
- As the water level in the tank approaches the pre-established full level, the pilot begins to modulate causing the main valve to throttle in a controlled manner and close hermetically.
- The adjustable pilot needle valve  ensures smooth operation of the main valve by regulating the closing speed. The test valve  allows manual opening of the main valve.