Just found two items while doing a different search for other EFI fuel issues.
1. You can buy a separate in-line fuel check valve from a number of manufacturers & retailers. It is exactly for this purpose.
Here's the text for one valve.
"Arrow marked on valve, showing fluid flow direction, Prevents fuel from returning back to the tank and keeps it in the line.
3) Also great for water pressure pumps & galley pumps which are not self-priming
4) Professional grade non-return valve - Ideal for marine, automotive, caravans/RV, and agriculture"
INHDBOX 8mm 5/16" Fuel Non-Return One Way Check Valve Petrol Diesel Marine Aluminum Alloy - Auto supplies depotAuto supplies depot
From a Porsche site in a discussion about adjustable fuel pressure regulators. There is a Bosch spec about how long the fuel pressure regulator should hold pressure after shutdown.
"Porsche specs for that series (87MY and up M28.41/42/47/49/50) are as follows:
Pressure at engine standstill (relay bypassed): 3.8 +/- .2BAR
Check value at idle : 3.3BAR
Minimum pressure after 20 min shut down : 3.0 BAR
Porsche P/N : 928.110.198.02
Bosch P/N : 028.160.262"
Bosch# for 3bar L-Jet type FPR? - Rennlist Discussion Forums
So the inline check valve thingy sound good to me, as I strongly suspect it's the check valve in your existing fuel pump that isn't doing it's job.
As to the gas pump wiring, well my car is different than most. First up I don't use a key to start the car. Furthermore the lock cylinder in the steering column is entirely removed and the wiring that goes into it has being moved into the dash. This was done by me, not the PO.
There's a home made plastic plate where the radio normally sits (I don't have a radio) in the plate sit two switches. A small toggle type switch and a larger spring loaded starter type switch. To start the car you flip the toggle switch to on, and press the starter switch till the car starts. To turn the car off, flip the toggle switch to off.
Since I hate keys, this suits me fine.
Hope this helps