This hydraulically operated Pressure Sustaining valve from Netafim is used to maintain a set pressure upstream of the valve. Three way manual switch is used to control operation and bleed any air from the valve upon installation. Pilot valve is used to set the desired pressure and then the three way manual switch would be turned to Auto and any pressure fluctuations in the lines upstream of the valve will be automatically monitored by the fast acting pilot valve which will activate the Pressure Sustaining valve diaphragm to adjust as needed to maintain set pressure.