diesel tank breather pipe???

[Boagie]

hey guys,i am in the process of changing my diesel tanks over.i was curious to find out about the small breather pipe(it is shaped like a candy cane)---if it should be able to be blown through--is it supposed to be clear --going into tank?.i am wondering if i need to get compressed air to blow it through or it is a one way valve coming out of tank.any info will be greatly appreciated

[Finly Owner]

They are designed to allow excess fuel to leave the tank when over full. I believe they would not be just hollow or they would act like a syphon, I may be wrong. I think some sort of pressure system is active when fuel cap is back on.

Anyone???????