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]

I like to think of S as "supply", D as "delivery", and R as "return". Easy to remember that way.

Teeing both the "R" lines from the pumps into the line returning from the regulator to the tank is the right thing to do, unless you want to make additional fittings on the tank.

I question the sense of sucking with two pumps through a single 3/8" fitting on the tank, as well as returning most of the fuel from both pumps and the excess from the fuel rail through a single 5/16" fitting.