Human lungs can only just blow up balloons. And the pressure inside a balloon is like 1 psi if that. Try drinking through 3 straws stuck together, it will be almost impossible.I wouldn't be surprised at all that you can't blow diesel back into the tank.
If the line isn't actually cracked sometimes you can take a small file to the line end and clean it up a bit. If the compression nut is overtightened it can mushroom the line, after which it won't seal too well. New lines will fix it for sure, but I'd try to fix the one I had first personally.