1969 1/2 ton 6cyl fuel pump problems

[l79m20]

My truck is having some fuel pump issues. All 3 of them are having issues. I went out and bought a new pump and installed it. My gas tank is 1/4 the way full. When I crank the truck over it will not pick up the gas and send it out the out valve of the pump. The gas has been taken out cleaned and the sending unit has been cleaned. I replaced the float on it. The lines from the sender unit is perfectly clear. The line from the fuel pump to the gas tank is perfectly clear. I can suck on the hose to the fuel pump and get gas from the tank. As soon as I connect it back to the pump, the pump wont suck it. I have tried 3 different fuel pumps and not one has worked. The pump sits with the hose connector on top.

WHAT IS THE PROBLEM WITH MY FUEL PUMP. PLEASE HELP ME!!