It should be a mechanical pump if it's first gen pickup.
You've got your supply, return and vent lines from the tank the one that goes to the charcole canister is just a vent the other two are supply and return. The pump has 3 lines one for fuel intake one for sending fuel to the carb and one for return.
if you're tanks only about half full you can tell which is which by blowing on the lines from the pump end the one that just dumps gas and blows smooth air is the return (returns extra fuel from the pump and pressurizes the tank) the other one will blow all the gas in the line back into the tank then start bubling like blowing into a drink with a straw.
If I remeber it right the pump is set up with 2 stacked lines and one solo line, I'm pretty sure the supply line is the lower of the stacked lines, I think the line to the carb is solo line ther other one above the supply line is the return line but I'm not sure those I might have mixed up.
Also pressurize the tank and try and listen for air leaking out, if it's signifigant enough it will keep your gas from reaching the pump, I went through a whole bunch of crap getting mine to work once and ended up having a leaky gas cap be the problem.