your truck wouldn't run at all without a FP regulator, but its built into the fuel metering body which is housed inside the intake manifold, like draggin mentioned....you didn't mention how much actual time it was taking to bleed off or what the circumstances are, but if the pressure is bleeding off quickly (right after key on and the fuel pump stops priming), then it could be the regulator allowing it to bleed off, it could be the metering body leaking it into the manifold, it could even be a leaking poppet or something....however no regulator is perfect and going to hold full pressure for long periods of time, if the issue is only happening when you're letting the truck sit for an extended period after key on engine off fuel pump priming, I wouldn't consider that anything to be worried about. the fuel injecton system is very sensitive to fuel pressure and requires it to be high to function. if the FPR is bleeing off a few psi over like 5-10 minutes of sitting after the fuel pump priming, I wouldn't consider the regulator in need of repalcing, but it would require re-priming, or a slightly extended cranking to get the pressure back up so the poppets will work....however if it is bleeding off over long periods of time, you might want to watch it, and check it every once in a while, and if it's getting worse, then that would be an indication that part of the regulator/poppets/etc is problematic and should be replaced.
the best thing to do is get a fuel pressure gauge, hook it up to the test port on the fuel rail and watch the pressure while the pump is priming see what it peaks at, and how long it takes for the pressure to drop.
