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Maybe there isn't a check valve in the system, and its designed that way to allow pressure to bleed off when the pump is not energized (ie. when you turn off the ignition key.) Maybe thats a safety thing, so that the fuel system is not constantly pressurized when the vehicle is off? Maybe that increases the life of your injector heads/seals , since they are only pressurized when the key is on? ie. not 24/7
I did a lot of research on this and MOST cars maintain fuel pressure. Then I found out that SOME Walbro pumps have a check valve and some don't. I don't think it matters as long as you allow the system to pressurize before cranking, which I usually do. I'm not going to do anything at this point and the long cranking (it's not that long) may be due to something else, like timing.