Petrol tank under pressure, blowing petrol lines out?

I recently replaced the petrol lines in my blower, and ever since, after about 30 mins of use, one of the petrol lines blows out of the tank and sprays petrol over my leg. It seems like the tank is pressurised, and I think it's a petrol return line into the tank that comes out. (It just goes through a friction fit hole in the bottom of the tank, with no filter or anything on that line.) It seems strange that the tank is under pressure, but I noticed it today when I took off the lid to put in more juice, and it hissed at me. I was initially thinking that I just had to somehow fit the line more firmly into the tank, but now I think the tank shouldn't be under pressure in the first place. Any ideas?