You have two valves in the FDM. One is the hi pressure check valve that prevents gas from entering the tank from the other fuel pump and hold the fuel pressure after you turn off the key.
The other valve is the shuttle valve and it prevents gas from entering the tank from the fuel pressure regulator when the other fuel pump is running.
The extra hi pressure check valve out side the tank will only work if the hi pressure check valve is bad but will do nothing for you if the shuttle valve is bad.

The FDM.


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