This weekend, I replace my rear nonfunctional pump and rusty up tank in my 1991 ford f150 4.9l.... The new tank and pump went on and hook up in no time. The rear tank is now functional but when it is running on the rear tank, gas come pouring out of the front tank so I guess it is the check valve? which I can't find where I can buy a factory check valve anywhere? There is one on each pump of my truck, would that be my problem? Also when I run it on the front tank there seems to be a lot of pressure that build up the tank when I take off the gas cap. what is the deal with that? I drive the truck everyday 80 miles, four days a week, so I kinda of need my truck running properly

 

Factory check valves are in the fuel delivery module which is inside the tank, so you need to change the FDM in the front tank.

Pressure inside the gas tank means your tank vent system isn't working correctly, both tanks should be connected to the charcoal canister in the engine bay on the passenger frame rail below the rad, which should then be connected to the throttlebody via the canister purge solenoid.