Why don't I have Fuel Pressure in my Ford F-150 pickup?

My 1990 Ford F-150 has duel tanks and fuel injection. The engine is 5.0 liter. It has two fuel pumps, both of which work.The fuel pressure should be around 30psi, but its much lower. I have replaced fuel filter, and relay, and even the rear tank. What else could it be? Is there a sensor or regulator for the fuel pressure? Right now, I can start the truck, and it will run for thirty seconds or so, then it dies and won't start again. What am I missing?