The burner fuel system is more complex than the pilot fuel system.  Kerosene from the supply tank at the rear of the vehicle is pumped with a piston pump in the pump box to a pair of tanks called service tanks.  These 1-1/2 quart tanks are plumbed in series with a working pressure of 140 PSIG.  The tanks are generally made from 14 gauge material.  Originally drawn from brass, the tanks were made of steel by the time condensing cars were introduced since the brass tanks were found to stress crack with the constant pressure cycling.  Today the tanks are available as copper reproductions since steel tanks were found to be prone to rust out due to moisture in the pressurization air condensing out and collecting in the bottom of the tanks.

A cushion of air in these tanks allow the pumps to fill the tanks with fuel while the air compresses thus building fuel pressure.  Kerosene fuel is pumped into the bottom of one of tanks (right one in the piping diagram and photo) while air can be admitted to the second tank (left tank in the piping diagram and photo).  This arrangement allows the fuel pumps (hand or power) to pump fuel against the air pressure in the tanks and pressurize the burner fuel supply system.

If the tanks are empty of fuel and air they are initially charged by pumping the hand fuel pump such that 10 or 15 PSIG is displayed on the burner fuel pressure gauge.  This step allows the fuel system to become primed since the pumps don't function well if there is air trapped in the lines or the pumps.  Once the pumps are primed as indicated by the burner fuel system beginning to build pressure, the tanks can be charged with their initial air cushion.  Using the air pressure valve for the burner fuel system the tanks are charged with air until the burner fuel pressure gauge shows the burner fuel system pressure to be around 90 PSIG.  After the air charge the hand pump is then used again to raise the burner fuel system pressure to nearly 140 PSIG.

As the steam automatic regulates the fuel supply to the burner to maintain steam pressure within the boiler, the air pressure of the left service tank in the diagram is what pushes the kerosene stored in the right service tank in the diagram to the burner.  Because the power fuel pump only operates when the car is in motion, the air in the service tanks is the only means to move fuel to the burner while the car is at rest such as at a stop light.  When the car is in motion the power fuel pump replenishes the fuel that was removed from the service tanks while the car was as rest.  For those times when the steam pressure is at its proper level and the burner is not firing, and the car is in motion and thus fuel is still being pumped (such as when descending a long hill), the fuel pressure automatic redirects the pumped fuel back to the supply tank once the pressure of the service tanks reaches 140 PSIG (the typical setting of the fuel pressure automatic).  When initially firing up a Stanley a hand pump is used to manually pump fuel from the supply tank to the service tanks to maintain fuel pressure for the burner.

During the operation of the car, over time, some of the air contained in the service tanks will diffuse into the kerosene much the same way carbon dioxide is diffused into carbonated sodas (it is the releasing of the carbon dioxide from the soda when it is poured in a glass that causes the bubbling action).  The air diffusing into the kerosene results in a decrease in the volume of air contained in the service tanks.  It is the air that provides the elastic properly within system and allows the hand or power pumps to actually build and maintain fuel pressure.  Reducing the volume of air in the tanks increases the tank volume available for the pumps to store fuel in.  Unfortunately the reduction in air space also diminishes the elasticity of the system such that each stroke of the power or hand fuel pump now has less air to push against resulting in the tank pressure building faster.  This is observed by the fuel pressure gauge needle making larger and larger motions or starting to "bounce". 

As the volume of air in the tank decreases it results in each stroke of the hand or power fuel pumps building fuel pressure faster. This can be observed on the fuel pressure gauge as the needle moving more with each pump stroke.  In a like manner when the steam automatic starts fuel flowing to the burner a continuous amount of fuel is removed from the service tanks resulting in an increasing drop of fuel system pressure due to the smaller volume of air present in the service tanks.  Thus the fuel gauge needle "bounces" such that each pump cycle increases the pressure while the burner operation is working to drop the system pressure.  When the bouncing motion of the fuel pressure gauge needle is observed it is an indication that air must be applied to the service tanks.  This is best accomplished the next time the car is fired up. 

Once the burner is firing on kerosene during the firing up operation, fuel is being removed from the service tanks and the service tank pressure will drop accordingly.  Normally the hand fuel pump would be used to replenish the fuel in the service tanks if the car is not in motion.  Using the hand fuel pump would both replenish the fuel supply of the tanks as well as build pressure in the service tanks.  However since they are low on air, connecting an air source to the air valves and opening the right air valve (right valve in the photo above; red valve in the piping diagram) is all that is necessary to add air volume to the service tanks and thus increase the burner fuel pressure.