Pressure sustaining valves or back pressure valves are designed to maintain a steady preset upstream / back pressure regardless of downstream pressure changes caused by increases in downstream demand. The pressure sustaining valve will stay open until the inlet / upstream pressure drops down close to the minimum pressure setting, at which point the pressure sustaining valve will slowly close to maintain its back pressure. When the back pressure returns safely above the minimum pressure setting the pressure sustaining valve will slowly open again allowing flow downstream. Each pressure sustaining valve can be chosen according to minimum and maximum pressure adjustment range, port size and material of construction to suit just about any media application. The upstream / back pressure is displayed by the pressure gauge.