This is a 99 chevy 5.7 3500 4x4 dump truck VIN 1GBJK34R7XF008777 single fuel tank gas, I have had two fuel tanks collapse like someone squezzed them in the middle. I get no back pressure when removeing the fuel cap and no check engine lights and no service codes. This is refered to as the old style with no pressure pressure sensor at the fuel module. It has like a 1/2 " hose going from the top of the fuel tank to the filler neck which was all clear and open., any ideas to what is causeing this as it's getting expensive. Note this is actually the 3rd tank, the 1st was replaced due to corrosion at the seams, but wasn't collapsed, I bought the other two tanks aftermarket from a radiator shop.