Electrical Dying Starter? Or Something Else?

After heading to the local O'reillys and nabbing their fuel pressure test kit, it looks like I'm losing pressure after the engine is shut off. Pressure is stock and stable while the engine's running, so essentially the problem is that I'm losing my prime when she sits for any amount of time and the pump isn't building enough pressure with just the single key turn. My local mechanic is recommending a one way check valve to keep the fuel from running back to the tank, but I'll be testing all the hoses for cracks and such too. Sad part is, the Mustang can't even sit for 10-15 now without having issues firing up so whatever it is is just getting worse.