fuel pressure automatic located under the floorboards just behind the front seat at the right side of the frame

As the burner does not fire all the time, especially when traveling down a long incline when no fuel pressure is required, the fuel pump would continue to pump fuel even though the burner is not consuming it. The pressure in the system would ultimately get so high that a fuel line or another component of the fuel system would rupture spraying kerosene at high pressure everywhere. The fuel pressure automatic sets the upper pressure limit for the burner fuel system thus preventing damage from overpressure situations. The fuel pressure automatic is set to limit the burner fuel system pressure to 140 PSIG.

HOW THE FUEL PRESSURE AUTOMATIC REGULATES FUEL PRESSURE

At the top of the fuel pressure automatic is a beryllium-copper disk or diaphragm.  This diaphragm is clamped between the two halves of the shell of the fuel pressure automatic (461 & 462).  The underside of the disk rests against a washer (468) which helps to keep the heavy spring (466) centered while fuel pressure is admitted to the top side of the disk.  An adjusting screw (464) and washer (467) allows for the spring tension against the disk to be set such that the diaphragm doesn't start moving and compressing the spring until the fuel pressure reaches 140 PSIG.

Mounted to the opposite side of the disk is the valve stem (465) which follows the position of the diaphragm.  The valve body is an integral part of the fuel pressure automatic's shell (462) as shown in the diagram.  Kerosene (gasoline for the early model Stanleys that used gasoline for both the pilot and burner) from the main burner pressure tanks is piped to the right port of the burner fuel automatic.  A line connected to the burner (main) fuel tank is connected to the port at the top of the fuel pressure automatic.

Normally the valve stem (465) is pressed tight against the valve seat (462) and no fuel passes from the right port through to the left port.  When the fuel pressure exceeds 140 PSIG (the normal set-point of the fuel pressure automatic) the diaphragm overcomes the spring force against it and it moves slightly (down in the diagram) to compress the spring (466).  As this motion happens the valve stem (465) moves off the valve seat (462) opening up the passage between the right port going to the fuel pressure tanks and the main fuel tank.  This action allows fuel to flow back to the tank and for the system pressure to be maintained at 140 PSIG.

When the fuel pressure pressure drops the spring force overcomes the fuel pressure acting on the disk and the diaphragm moves such that the valve stem (465) returns to being seated and thus fuel will not pass though the automatic.  During normal operation the majority of the fuel pumped by the power fuel pump is returned through the fuel pressure automatic to the fuel supply tank.

Careful review of the fuel piping system also will reveal that when the hand fuel pump is used the fuel pressure automatic also limits fuel pressure.  Generally when firing up a Stanley and using the hand fuel pump it is only necessary to keep the burner fuel pressure at no more than 130 PSIG as indicated on the fuel pressure gauge.  Pumping the pressure much higher only means that most of the effort being put into pumping fuel is being wasted as it is being returned to the supply tank by the fuel pressure automatic.