I recently replace the fuel tank on my 1968 roadrunner and put in a new sending unit. Now when I fill the tank more then half full and step on the brakes gas runs out the vent line(the one that curls around the axle). I used all the parts(nothing left over) but it seems like there should be a bladder or check valve that goes on the vent line. Anyone have experience with something like this or know where I can get the proper parts. Ive had no luck finding something online.

Is this a new tank or are you using a different cap? If it's a stock tank with the stock hoses and they aren't leaking at the clamps it's possible you may have the problem I have seen twice. Once on a brand new 1969 Roadrunner and once on a 1969 GTX I bought that I think had the original tank someone tried gluing. In my case the vent tubes were leaking right where they exited the tank due to poor tank assembly. In both cases the tank was replaced.