confusing problem at the pump

Well, I know that the price of gas is really out of control but what about actually getting the gas into the tank??? I drive a 1978 chevy 1/2 ton fleetside pickup and my father drives a 1976 of that same truck. There is a problem when we go to fill them up at the pump though.... when i put the nozzle in to the filler neck and start filling, the tank pressures up and blows most if not all of the fuel onto the side of the truck and the ground. The confusing part of this is that I checked the line running from the tank to the charcoal vapor recovery filter under the hood of the truck that should serve to releive the pressure and prevent this from happening but it is not plugged. The fact that it does this on both trucks leads me to believe that this is a common problem, yet I cannot find a solution on the internet thus far. Is anyone else having this problem?? I can put fuel in a gas can and pour it into the tank like that and it does fine. Any help on this would be greatly appreciated.  
1978 Chevy Pickup receiving a small block 350 built by me

What state do you live in? I know out here in CA we have these goofy things on most of the fillers that keep the air clea and you have to tip it in right or it will make a mess.  

I had a similar problem with my 1985 GMC sierra p/u. I never did find out what the cause was but I always thought it had something to do with the ANGLE of the filler tube. maybe there is a baffle at the end of the tube ( right at the gas tank ) which when filling , might cause BLOWBACK, because I found that when you slowed the gasoline down going in, the problem disappeared.  

Yeah, i would try not pulling the trigger all the way, so it slows down the flow, like screw said.  

The filler hose has a smaller inner hose in it. This is what the fuel goes through. The lager outer hose is for the vapor to escape when you are filling. If the the inner hose collaspes or has a large hole in it then the fuel & the escaping vapor fight for the same space. This will cause the fuel to backflow. I would take the filler hose completely off & look inside of it.  

I would suggest not smoking or using a cell phone while filling up!  

I have a 76 c10 with the same problem what I did was cut a little bit of the rubber line to give it more pitch the other thing that seems to work is flipping the pump handle upside down and keep the handle tipped close to the truck bed.I hope that helps!