Fuel pump physics, Push/pull/suck/pump? Options

[lmcchesney]

My little fuel pump(72) has been working for six years without a problem. Now that I am rebuilding the car and looked at the fuel hose diagram with fuel flows as per PP and DD, I must admit. I have not the foggest how this little pump works.
Fuel tank supply feeds a "Y" connector with the "R" port on the pump and the fuel line feeding the engine compartment. I do not see a valve in the "Y" connector.
Fuel returns from the engine compartment to enter the "S" port. Then the "D"port returns the fuel to the tank.
If the roller pump works by pushing fuel out the "R" port, why would the fuel not flow to the engine compartment and back to the tank via the supply line?
I have done a search and reviewed the classic section without finidng the answers.
What mechanism is being used?

Thanks,
L. McChesney
lamcchesney@netzero.net

[ChrisFoley]

L.,
Twice the pumps equals twice the volume. There is no feedback loop to tell the pumps they don't need to pump continuously. The fuel pumps will both bypass all the fuel they don't send through the injection rails and it will have to go back to the tank through the "return" line. While the amount of fuel flowing through the injection system may not increase greatly, the amount exiting and re-entering the tank will roughly double from the stock configuration. I am concerned that a pressure drop or backpressure at the tank fittings will negatively influence the gains you hope to achieve by running two pumps. It doesn't matter how big the injection rails are. The point of greatest restriction will be at or near the tank.