The round cross member in front of the gas tank on my friends sidekick rotted out and his tank fell out. He was really lucky that it fell off in the parking lot at his work. Yesterday I ground the frame rails down, welded plates over the holes where the the old bar was and welded a new piece of square tubing in. At the back I did the same thing and did the gas tank lift since I already did the 3" body lift a month ago for him. It worked out really well, but after I hooked all the lines up the little plastic canister on top of the tank started leaking gas out from the seam.
 What's the purpose of this little canister, I didn't pay too much attention to it until it started leaking. I'm wondering, if I put those two small lines on backwards(the ones that run along he passenger side of the frame) would this cause an issue? It was towed to my shop with the tank in the back so I didn't see them come off. I will look today at mine to see how they go on, but I really would like to know what that plastic thing is for?
Thanks

Mine doesn't have anything there, but I've been told it's a filter. I only have 2 lines running to the tank, pressure and return. there is a third line, but whoever had the truck before decided it wasn't needed. It runs fine, but who knows what damage it's slowly causing.

On my 8v ihad the same problem gas tank fell out rotted cross bar when fixed the bar we lifted the tank and he removed the little plastic pc said it held fumes part of the charcoal canister dont have that so i did not need the plastic case damn emmisions sh#%....

Its a vapor/gas seperator, meant to ensure that only fumes get to the charcoal canister.

Thanks for the replies
So if it's leaking alot of gasoline, I mean it pissses out everywhere, does that mean that I have connected those two lines incorrectly? I think that must be it if it's just for fumes, because it looks like Niagara falls with so much gas pouring out of the seam!!!

Just took a look at mine take th plastic case off and loop the hose from one nipple to the other never had a problem with it

Thanks CJ11, I'll give that a try!

I removed a rusty body today here is a pic of stock hookup 89 tracker
http://www.fotothing.com/rocky99/

I am missing that same cross member pipe.  Luckly it rotted off in the middle so my gas tank is still ok.  But I better get on that before I lose mine.

I should be thankful for how rust free mine is. When living in california, you kind of forget the horrors of rust and how it can destroy a vehicle in a matter of 10 years.