A pressure regulating and pressure releasing valve, comprising: (a) a valve body defining an inlet port, an outlet port, a valve seat and a pressure release opening; (b) a pressure regulating plug displaceable from an open position, which provides a continuous flow path from the inlet port to the outlet port, to a flow-throttling position in which the plug is at least partially closed against the valve seat to obstruct the continuous flow path, the plug being further displaceable beyond said flow-throttling position to a pressure-release position in which a pressure relief flow path is opened from the outlet port through the pressure release opening to an area external to the valve body; and (c) a spring configured to bias the plug from the pressure-release position to the open position, the plug having an actuation surface exposed to a pressure at the outlet port such that the pressure at the outlet port displaces the plug against the bias of the spring toward the closed position, thereby variably throttling the continuous flow path to achieve outlet pressure regulation, and wherein increased pressure at the outlet port causes further displacement of the plug beyond the flow-throttling position towards the pressure-release position, thereby opening the pressure relief flow path from the outlet port through the pressure release opening, characterized in that the valve seat comprises a first cylindrical wall and the plug comprises a second cylindrical wall, displacement of the plug from the open position to the flow-throttling position being an axial motion of the second cylindrical wall towards, and coaxially aligned with, the first cylindrical wall, and wherein displacement of the plug to the pressure-release position brings the first and second cylindrical walls into overlapping relation. (a) a valve body defining an inlet port, an outlet port, a valve seat and a pressure release opening; (b) a pressure regulating plug displaceable from an open position, which provides a continuous flow path from the inlet port to the outlet port, to a flow-throttling position in which the plug is at least partially closed against the valve seat to obstruct the continuous flow path, the plug being further displaceable beyond said flow-throttling position to a pressure-release position in which a pressure relief flow path is opened from the outlet port through the pressure release opening to an area external to the valve body; and (c) a spring configured to bias the plug from the pressure-release position to the open position, the plug having an actuation surface exposed to a pressure at the outlet port such that the pressure at the outlet port displaces the plug against the bias of the spring toward the closed position, thereby variably throttling the continuous flow path to achieve outlet pressure regulation, and wherein increased pressure at the outlet port causes further displacement of the plug beyond the flow-throttling position towards the pressure-release position, thereby opening the pressure relief flow path from the outlet port through the pressure release opening, characterized in that the valve seat comprises a first cylindrical wall and the plug comprises a second cylindrical wall, displacement of the plug from the open position to the flow-throttling position being an axial motion of the second cylindrical wall towards, and coaxially aligned with, the first cylindrical wall, and wherein displacement of the plug to the pressure-release position brings the first and second cylindrical walls into overlapping relation.