When the pressure of the fluid in port 1 exceeds the preset value, piston 2 is pushed to the right, overcoming the resistance of spring 6 and allowing the fluid to flow through port 3 to the tank. To improve the sensitivity of the valve, allowing it to operate at a lower pressure, the pressure is raised in chamber 4 through port 5. The higher the pressure in chamber 4, the less the pressure required in port 1 to shift the piston.
$3619$SHP,Va$