Well to start with, since the 4BT uses less fuel than a 6BT and you can't get fuel with a lowly mech lift pump, you obviously have a plugged line or pickup or perhaps a metal line on frame rail is crushed. I run standard 3/8 rubber lines from my tank to electric lift pump on a 6BT through a factory pickup in tank aside from modifying the pickup tube assemble inside the tank by removing the factory hard plastic line from the bottom strainer to the cap using more standard 3/8 line and brass fittings threaded into plastic. I have never seen a factory style mech lift pump make enough vacuum to collapse a rubber line before, so it appears to be your injector pump primary is doing a real good job. Frankly I would trash the mech pump for a good elect. lift, if you run out of fuel in the tank, you will certainly wish you had an elect. lift mounted near the tank. I have seen rust get in lines and make a plug from gelled fuel in diesel and gas before. Disconnect from filter inlet side and use low air pressure to see if you can blow back into tank. Always a good idea to pull tank and refresh line system to be sure you will not have to deal with dirty fuel for as long as possible in a situation like this if the vehicle has been stationary for long time. Right now I'm dealing with a no fuel issue on my 6BT also. Only difference is it will start up and run fine for awhile then starve for fuel, once that happens, I can't get re primed. That's with electric lift and so far I haven't found any prob with lift or pickup or filter or lines. I ran perfect the day before when I was pulling 30k lbs. through hills and 7 % grades.I recently did a 4bt swap with the VE injection pump and a mechanical lift pump. I am having trouble starting with the fuel lines I used because they’re collapsing. Does anybody have a good recommendation for fuel lines that won’t restrict my flow? I’m running 3/8 line.