Fuel problems

[DET45]

A friend and myself own a 1978 and 1979 chevy pickups. His P/U has a fresh 350 and mine has a fresh 454. Both of our trucks seems to lose fuel out of the fuel line when our trucks get to operating temperture. When we first start up our trucks our glass fuel filters fill up with gas and the trucks run fine. However after they warm up the filters start to drain and my fuel pressure gage drops to zero. Truck still runs but goes flat when driving. I have changed fuel pumps three times to include electric pump, edelbrock mech pump, and factory pump, I even used the electric and mechanical pump together and yet the fuel problem still exists. I also changed three different carburetors. After that I dropped the fuel tank and checked out thr fuel sending unit and for any obstructions (none) So I replaced all of the fuel line, still losing fuel. After that I ran the fuel line in a gas can to eliminate the trucks gas tank (still loses fuel) I checked for vacume leaks on the intake and other areas (ok?) My gas tank has one fuel supply line and one vent line, both are new and un-obstructed. I could be wrong but it looks like the fuel is being pushed out and down the fuel line from the carburetor. The reason why I say this is because I have my fuel system set up starting with the carb, then the glass fuel filter, then the fuel pressure gage, pump, then gas tank. What happens is after the truck reachs operating temperture you can see the glass fuel filter (located approx 6 inchs from carb) starting to drain then after that the fuel pressure gage (down stream from fuel filter) starts to drop to zero. That tells me that the fuel is being pushed out and down the fuel line I think? Anyway I am totally confused and don't know what else I can do. My friends 79 P/U is doing the same thing and we are both are scratching our heads. PLEASE HELP......Thank You....