The injection pump, I'm guessing. Considering I had zero problems before switching, it must be something directly related to the Holley conversion, not an IP problem. Doesn't the filter head return line connection have a check valve? Perhaps it's weak and is bleeding pressure from the supply. Just a thought. The fuel pressure drops when shut off, as if it's going somewhere. Wouldn't it normally hold pressure? Maybe it's how Holley pumps just are. I had no starting problems with the mech pump so it wasn't losing prime.