I have a 1983 f-250 with a 7.5L. It has dual tanks with in tank electric fuel pumps. I am only getting 2 psi from each tank when running off the truck electrical. I tested the output of the pump connecter and it read 8 volts. This is not enough to keep the bowl full at the carb.

I ran them straight from another battery using jumper cables and they ran at around 5-6 psi. This was enough to overflow the carb.

What is causing the low pressure?

I read in my Haynes manual that the power output at the tank connector should be around 10.5v.

what is causing a lower voltage at the pump connector?

 

I think you know what is causing the low pressure - low voltage. So the real question is why is the voltage so low. I don't have any wiring diagrams for trucks with in-tank pumps, so I don't know what all is in the circuit. But obviously something is giving you a poor connection. I assume the switch on the HVAC panel switches both gauges as well as pumps. I would start checking voltages there. Note the color of wire that goes to the pumps and find those wires on the switch and check the voltage on the connectors. If it is battery voltage or close then you have a problem between there and the tanks. But, since it isn't likely you have two problems that give the same voltage, I'll bet the switch is only getting fed ~8v. If that's the cause then you'll have to trace the wire to find out where it goes, but that is very difficult. You really need wiring diagrams, so hopefully someone will come along that does and post them for you.