My vehicle:
1976 F150 2wd Extended cab
390 4bbl
c6 transmission
4BBL Edlebrock Carb
Dual fuel tanks, only rear main tank connected

My problem:
For the first year I owned the truck, everything ran fine and it started up really quickly even after it sat for a few days or a few weeks.
I have since, for about the last year or so, had to crank it for 30-45 seconds to get it to start. This all started around the time my fuel pump went out. I replaced it with an Airtex fuel pump from Autozone. I also installed a glass in line filter before the carb in front of the distributor.
I noticed that after about 4-6 hours, the fuel had drained out of the filter and that's when I would have to crank it forever to get it going. It also seems like there is no fuel in the carb as it doesn't even try to start when it first gets cranked.
I thought this was a bad pump, so I bought a new one last week and tried it again. Sure enough after about 4-6 hours, everything is empty again and it takes forever to start it. So it's not a bad pump, or so I think.
The other day I backed it into my driveway and left it, something I never do. I checked about 4 days later and there was still fuel in the filter. This makes me think that gravity is making it drain back to the tank.

Is there something that would cause this? Also I have found a one way fuel check valve online that is supposed to remedy this type of problem.
Has anyone used one of these before? Do they work for this situation? Or is there something else I should be doing to fix this? It has taken a bit of a toll on my starting system having to crank so long all the time. I have so far had to replace my starter, starter solenoid, and all the battery cables, starter cable, terminals, etc. due to excessive wear.

The valve I found is here.
http://www.ebay.com/itm/Aluminium-Fuel-Non-Return-Check-Valve-One-Way-Petrol-Diesel-6mm-8mm-10mm-12mm-/390801413623?pt=LH_DefaultDomain_0&var=&hash=item5afd9455f7

 

is the tank that you do not use empty. how do you know the fuel is draining back the sight filter does not tell you much as a normal fuel system would allow some fuel back into the tank.next cold start take the air cleaner off and look down the air horn of the carburetor work the throttle and see if fuel is squirting down into the intake. what pressure is the pump you are running.

 

The tank I don't use isn't even hooked up. My tank switcher broke on of it's outlets off, so it's just straight hose from the rear main tank to the fuel pump.

It is draining all the fuel out of the fuel line to the point that the fuel pump isn't even primed. I have to crank it for about 30-45 seconds, sometimes longer to get fuel up to the carb.

I have looked down the carb during a cold start, and fuel is not squirting down into the intake. It's dry on cold starts, not even any gas left in the bowl. I'm thinking it might be either boiling out or possibly the suction from it draining back from the tank is siphoning it out.

I'm running a standard mechanical fuel pump, about 5.5-6.5psi.

 

well have you done a pressure check on the system. if their are any leaks at all between the pump and the float bowl then it makes it very difficult for the pump to build up pressure and supply the carburetor with fuel some leaks only occur when the engine is running some when the engine is off. these trucks do not run a return system so if fuel is draining most likely that would be a defective pump. the fuel boiling is unlikely on a cold start unless you think is could be vapor locked after cranking it because it sounds like its out of fuel i and it is non efi i also doubt that check for leaks and if you want to ease your mind block the line returning to the tank after the pump build pressure let it set then go start it.