The oil pressure regulator is actually a diaphragm-controlled overflow relief valve. A diaphragm separates the regulator into an upper spring chamber and a lower fuel chamber. And the opening of the relief valve is controlled by the diaphragm to maintain the pressure balance between the upper and lower chambers.
When the oil pressure in the fuel chamber rises beyond the combined force of the spring pressure and the vacuum gas pressure, the diaphragm arches upward, the regulator valve opens, and part of the fuel flows from the ball valve to the fuel tank through the return port, reducing the fuel pressure. When the control oil pressure is set by the regulator, the ball valve is closed to maintain a certain pressure in the oil circuit from the oil pump check valve to the pressure regulator.