A pressure regulator for a BOP stack includes a housing having an input port, an output port, and a pilot port and a regulator piston axially movable through the housing between an open position and a closed position. The regulator piston has an internal passage formed through its first axial end. When the regulator piston is in the open position, the internal passage is in fluid communication with the input and output ports. When the regulator piston is in the closed position, the internal passage is in fluid communication with the output port and fluidly isolated from the input port. A space with regulator fluid is also provided within the housing adjacent a second axial end of the regulator piston, opposite the first axial end, wherein the space is fluidly isolated from the input port and the output port in both the open position and the closed position.