Bad leak in 98 custom

 

I recently got a longbow air thru stock for my 98 custom. It told me to change my gas tube to the one sent with it and that it would connect the air line to the stock. After adding it i realized that there was a leak in my custom. So i changed back the gas line to the braided one that came with my gun. After changing it back i have realized that there is a bad leak in my gun. I wanted to know what I could do to fix this problem.

 

Thanks