3406b air build up in fuel system

We pulled the motor from another truck and it ran fine now its in another truck it will run for a few minutes untill it sucks the fuel out of filter runs ruff and dies. We have noticed that there is a lot of pressure when we took the filter off and we are getting no air anywhere either. What's wrong?

 

I dont know if it matters but the motor is a 3406b. I usally dont have to mess with this stuff but what is a 1212? And the motor came out of an 86 and now is in a 96. We changed everything over from the old motor except the exhaust mainfold and we put a cross over for the fuel tanks thats it. I didnt put the motor back in someone else did but theres a line coming off the main filter going down to a valve which there is a line coming off air pump that goes down to the same valve, does ths sound right?

 

Ill try and post pics tommorrow. But if you are looking at main filter to lines come off it one goes toward the back of the truck the other towards bottom/front of truck and there's a line that comes off air pump and goes to the same area as the fuel line. On the pump I have 2 lines that go back to tanks it looks like one coming off the side going to intake? 1 coming off bottom going to front cover and the big one going to the area of the fuel line. The filter is a off 2749.

 

The black line from the pump is going down to this block which is at the bottom towards the front of motor and the small line is going to fuel filter. Ill post pics of the filter to.