I had to replace my sending unit with a new one.The strainer on the old one was deteriorated and fell apart in the tank and i would run out of fuel at 1/4 tank.While i was in there i could only cut off the anti siphon tube(the smaller one)near the fill up spout,i couldn't cut off the bigger one so i left it in there.Is it necessary to cut the big one off to because now i can fill up 35 gal of diesel,before i could only get 25 gal but i read that you have to cut both of them off.

 

Not necessary. I would not have cut the big fill tube off either, if i did it.. Just the vent tube is all you need to cut.

 

Cutting the fill tube is virtually of not much consequence at all... Dan's right... the vent tube is the critical path on that effort.